Meta LWG issue: 2025-11 meeting

[StephanTLavavej]

(Previous meta-issue: #5624)

At the November 2025 meeting, the following LWG issues were resolved in the C++ Working Paper.

❔ Not yet analyzed

• Remaining issues.
  • All done!

❌ Not applicable

If an issue requires no action from implementers, we mark it as N/A.

• Pure wording clarifications with nothing to implement (these can be changes to non-normative text like examples and informative notes, or wording cleanups to normative text that don't impact observable behavior).
  • LWG-4286 Some more feature-test macros for fully freestanding features are not marked freestanding
    • We're a hosted implementation.
  • LWG-4317 The meaning of "resource" in the Cpp17Destructible requirements is undefined
  • LWG-4377 Misleading note about lock-free property of std::atomic_ref
  • LWG-4440 Forward declarations of entities need "also in" entries
    • We define feature-test macros in all headers.
  • LWG-4462 Algorithm requirements don't describe semantics of s - i well
• Something that increases the restrictions placed on users, but implementers aren't expected to enforce those restrictions.
• Fixes for obviously broken wording, where implementers would have done the right thing anyways.
  • LWG-4451 make_shared should not refer to a type U[N] for runtime N

😸 Already implemented

Sometimes we cite LWG issues in product code comments as we're implementing their proposed resolutions. When the resolutions are officially accepted, we should remove the citations (as the default assumption is that we're implementing what the Standard says). If something is especially subtle, we can convert the citation to mention the relevant Standard section. Sometimes we should add test coverage, for example when the Standard begins requiring something that we were already doing but weren't explicitly testing for.

• Already implemented, comments need to be removed, and messages need to cite the Standard.
  • LWG-3454 pointer_traits::pointer_to should be constexpr
    • LWG-3454 pointer_traits::pointer_to should be constexpr #5864
• Implemented without comments.
  • LWG-2991 variant copy constructor missing noexcept(see below)
    • Took me a while to find, but I believe this was implemented without any "strengthened" comments:

      STL/stl/inc/variant

      Lines 818 to 819 in ab6dec6

      	_CONSTEXPR20 void _Construct_from(const _Variant_base& _That)
      	noexcept(conjunction_v<is_nothrow_copy_constructible<_Types>...>) {

  • LWG-4020 extents::index-cast weirdness
  • LWG-4265 std::midpoint should not accept const bool
  • LWG-4269 unique_copy passes arguments to its predicate backwards
  • LWG-4275 std::dynamic_extent should also be defined in <mdspan>
  • LWG-4276 front() and back() are not hardened for zero-length std::arrays
  • LWG-4370 Comparison of optional<T> to T may be ill-formed

🩹 Patches an unimplemented feature

• Recorded this LWG issue in the GitHub issue tracking the feature. That way, we'll remember to verify it, but it doesn't represent net new work.

• P1222R4 <flat_set> #2912

  • LWG-4384 flat_set::erase(iterator) is underconstrained

• P0792R14 function_ref: A Type-Erased Callable Reference #3788

  • LWG-4256 Incorrect constraints for function_ref constructors from nontype_t
  • LWG-4425 CTAD function_ref of data member pointer should produce noexcept signature

• P2548R6 copyable_function #3803

  • LWG-4255 move_only_function constructor should recognize empty copyable_functions

• P2630R4 submdspan #3807

  • LWG-4372 Weaken Mandates: for dynamic padding values in padded layouts

• P1673R13 <linalg>: Linear Algebra #4170

  • LWG-4136 Specify behavior of [linalg] Hermitian algorithms on diagonal with nonzero imaginary part
  • LWG-4137 Fix Mandates, Preconditions, and Complexity elements of [linalg] algorithms
  • LWG-4302 Problematic vector_sum_of_squares wording
  • LWG-4315 Insufficient specification of vector_two_norm and matrix_frob_norm

• P2447R6 Constructing span<const T> From initializer_list<T> #4173

  • LWG-4293 span::subspan/first/last chooses wrong constructor when T is const-qualified bool

• P2248R8 Enabling List-Initialization For Algorithms #4513

  • LWG-4444 Fix default template arguments for ranges::replace and ranges::replace_if

• P2542R8 views::concat #4514

  • LWG-4166 concat_view::end() should be more constrained in order to support noncopyable iterators

• P0843R14 <inplace_vector> #4766

  • LWG-4396 Improve inplace_vector(from_range_t, R&& rg)

• P2300R10 std::execution #4768

  • LWG-4260 Query objects must be default constructible
  • LWG-4328 Remove note in [exec.sched] regarding waiting for completion of scheduled operations
  • LWG-4340 task::promise_type::unhandled_stopped() should be noexcept
  • LWG-4341 Missing rvalue reference qualification for task::connect()
  • LWG-4342 Missing rvalue reference qualification for task_scheduler::ts-sender::connect()
  • LWG-4343 Missing default template arguments for task
  • LWG-4345 task::promise_type::return_value default template parameter
  • LWG-4346 task::promise_type::return_void/value lack a specification
  • LWG-4349 task is not actually started lazily
  • LWG-4358 [exec.as.awaitable] is using Preconditions: when it should probably be described in the constraint
  • LWG-4360 awaitable-sender concept should qualify use of awaitable-receiver type
  • LWG-4369 check-types function for upon_error and upon_stopped is wrong
  • LWG-4415 task::promise_type::uncaught_exception seems to be misnamed
  • LWG-4438 Bad expression in [exec.when.all]
  • LWG-4445 sch_ must not be in moved-from state
  • LWG-4446 Bad phrasing for SCHED(s)
  • LWG-4447 Remove unnecessary sizeof...(Env) > 1 condition
  • LWG-4448 Do not forward fn in completion_signatures
  • LWG-4455 Add missing constraint to basic-sender::get_completion_signatures definition
  • LWG-4456 Decay Data and Child in make-sender
  • LWG-4459 Protect get_completion_signatures fold expression from overloaded commas
  • LWG-4461 stop-when needs to evaluate unstoppable tokens

• P1928R15 <simd> #5141

  • LWG-4230 simd<complex>::real/imag is overconstrained
  • LWG-4280 simd::partial_load uses undefined identifier T
  • LWG-4375 std::simd::bit_ceil should not be noexcept
  • LWG-4376 ABI tag in return type of [simd.mask.unary] is overconstrained
  • LWG-4382 The simd::basic_mask(bool) overload needs to be more constrained
  • LWG-4403 simd::basic_vec CTAD misses difference type casting
  • LWG-4407 constexpr-wrapper-like needs remove_cvref_t in simd::basic_vec constructor
  • LWG-4412 Fix declaration of zero_element and uninit_element
  • LWG-4413 Unused/left-over simd::alignment specialization for basic_mask
  • LWG-4420 [simd] conversions (constructor, load, stores, gather, and scatter) are incorrectly constrained for <stdfloat> types

• P3508R0 constexpr For Raw Memory Algorithms #5148

  • LWG-4452 Make deref-move constexpr

• P3372R3 constexpr Containers And Adaptors #5296

  • LWG-4291 explicit map(const Allocator&) should be constexpr
  • LWG-4292 Unordered container local iterators should be constexpr iterators

• P0447R28 <hive> #5301

  • LWG-4318 Have hive::erase_if reevaluate end() to avoid UB

• P3019R14 indirect And polymorphic: Vocabulary Types For Composite Class Design #5302

  • LWG-4251 Move assignment for indirect unnecessarily requires copy construction

• P2996R13 <meta>: Reflection #5606

  • LWG-4316 {can_}substitute specification is ill-formed
  • LWG-4416 <meta> should include <compare>
  • LWG-4422 meta::access_context should be a consteval-only type
  • LWG-4423 meta::data_member_spec allows negative bit-field widths
  • LWG-4424 meta::define_aggregate should require a class type
  • LWG-4426 Clarify what meta::reflect_constant_string considers a string literal
  • LWG-4427 meta::dealias needs to work with things that aren't entities
  • LWG-4428 Metafunctions should not be defined in terms of constant subexpressions
  • LWG-4429 meta::alignment_of should exclude data member description of bit-field
  • LWG-4432 Clarify element initialization for meta::reflect_constant_array
  • LWG-4433 Incorrect query for C language linkage
  • LWG-4434 meta::is_accessible does not need to consider incomplete D
  • LWG-4435 meta::has_identifier doesn't handle all types
  • LWG-4442 Clarify expr and fn for meta::reflect_object and meta::reflect_function
  • LWG-4443 Clean up identifier comparisons in meta::define_aggregate
  • LWG-4449 define_aggregate members must be public

• P2988R12 optional<T&> #5607

  • LWG-4299 Missing Mandates: part in optional<T&>::transform
  • LWG-4300 Missing Returns: element in optional<T&>::emplace
  • LWG-4304 std::optional<NonReturnable&> is ill-formed due to value_or
  • LWG-4308 std::optional<T&>::iterator can't be a contiguous iterator for some T
  • LWG-4439 std::optional<T&>::swap possibly selects ADL-found swap

• P3348R4 C++26 Should Refer To C23, Not C17 #5608

  • Increases restrictions placed on users (IFNDR):
    • LWG-4388 Align new definition of va_start with C23
  • Reverts an overlooked silent change, no action required:
    • LWG-4430 from_chars should not parse "0b" base prefixes

• P3179R9 Parallel Range Algorithms #5610

  • LWG-4297 Missing permutable constraint for iterator overloads in Parallel Range Algorithms
  • LWG-4431 Parallel std::ranges::destroy should allow exceptions
  • LWG-4441 ranges::rotate do not handle sized-but-not-sized-sentinel ranges correctly
  • Wording clarifications:
    • LWG-4463 Change wording to 'model' from 'subsumes' in [algorithms.parallel.user]
    • LWG-4464 [alg.merge] Wording tweaks
    • LWG-4465 [alg.partitions] Clarify Returns: element

• P2830R10 type_order #5614

  • LWG-4305 Missing user requirements on type_order template

• P3111R8 Atomic Reduction Operations #5616

  • LWG-4450 std::atomic_ref<T>::store_key should be disabled for const T

• P2781R9 constant_wrapper #5619

  • LWG-4351 integral-constant-like needs more remove_cvref_t
  • LWG-4383 constant_wrapper's pseudo-mutators are underconstrained

🐞 Not yet implemented

• Filed a GitHub issue labeled LWG Library Working Group issue .
  • LWG-3090 What is [time.duration.cons]/4's "no overflow is induced in the conversion" intended to mean?
    • LWG-3090 What is [time.duration.cons]/4's "no overflow is induced in the conversion" intended to mean? #5850
  • LWG-3343 Ordering of calls to unlock() and notify_all() in Effects element of notify_all_at_thread_exit() should be reversed
    • LWG-3343 Ordering of calls to unlock() and notify_all() in Effects element of notify_all_at_thread_exit() should be reversed #5863
  • LWG-3627 Inconsistent specifications for std::make_optional overloads
    • LWG-3627 Inconsistent specifications for std::make_optional overloads #5851
  • LWG-4015 LWG-3973 broke const overloads of std::optional monadic operations
    • LWG-4015 LWG-3973 broke const overloads of std::optional monadic operations #5871
    • Also noted in P2988R12 optional<T&> #5607
  • LWG-4243 as_bytes/as_writable_bytes is broken with span<volatile T>
    • LWG-4243 as_bytes/as_writable_bytes is broken with span<volatile T> #5852
  • LWG-4253 basic_const_iterator should provide iterator_type
    • LWG-4253 basic_const_iterator should provide iterator_type #5853
  • LWG-4257 Stream insertion for chrono::local_time should be constrained
    • LWG-4257 Stream insertion for chrono::local_time should be constrained #5854
  • LWG-4266 layout_stride::mapping should treat empty mappings as exhaustive
    • LWG-4266 layout_stride::mapping should treat empty mappings as exhaustive #5855
  • LWG-4272 For rank == 0, layout_stride is atypically convertible
    • LWG-4272 For rank == 0, layout_stride is atypically convertible #5856
  • LWG-4274 The chrono::hh_mm_ss constructor is ill-formed for unsigned durations
    • LWG-4274 The chrono::hh_mm_ss constructor is ill-formed for unsigned durations #5857
  • LWG-4294 bitset(const CharT*) constructor needs to be constrained
    • LWG-4294 bitset(const CharT*) constructor needs to be constrained #5858
  • LWG-4301 condition_variable{_any}::wait_{for, until} should take timeout by value
    • LWG-4301 condition_variable{_any}::wait_{for, until} should take timeout by value #5859
  • LWG-4312 Const and value category mismatch for allocator_arg_t/allocator_arg in the description of uses-allocator construction
    • LWG-4312 Const and value category mismatch for allocator_arg_t/allocator_arg in the description of uses-allocator construction #5866
  • LWG-4366 Heterogeneous comparison of expected may be ill-formed
    • LWG-4366 Heterogeneous comparison of expected may be ill-formed #5873
    • Also noted in P2944R3 Comparisons For reference_wrapper #4522
  • LWG-4398 enable_nonlocking_formatter_optimization should be disabled for container adaptors
    • LWG-4398 enable_nonlocking_formatter_optimization should be disabled for container adaptors #5860
  • LWG-4399 enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t
    • LWG-4399 enable_nonlocking_formatter_optimization for pair and tuple needs remove_cvref_t #5861

[StephanTLavavej]

I'll do a first pass here to patch the obvious feature-issues; no need for anyone to look at this until I do that mechanical work.

[frederick-vs-ja]

Patching C++26 features:

• LWG-4166 patches P2542R8 views::concat
• LWG-4255 patches P2548R6 copyable_function
• LWG-4291 and LWG-4292 patch P3372R3 constexpr Containers And Adaptors
• LWG-4293 patches P2447R6 Constructing span<const T> From initializer_list<T>; but we can tweak implementation details in old modes to avoid the problem
• LWG-4351 patches P2781R9 constant_wrapper
• LWG-4372 patches P2642R6 Padded mdspan Layouts (tracked together with P2630R4 submdspan)
• LWG-4433 patches P2996R13 <meta>: Reflection
• LWG-4444 patches P2248R8 Enabling List-Initialization For Algorithms
• LWG-4450 patches P3111R8 Atomic Reduction Operations
• LWG-4452 patches P3508R0 constexpr For Raw Memory Algorithms

Patching C++26 features, but likely to be pure wording fix/no-op:

• LWG-4286 patches numerous C++26 freestanding addition papers; but we currently only support hosted modes
• LWG-4388 patches P3348R4 C++26 Should Refer To C23, Not C17; but perhaps only UCRT should change
• LWG-4430 patches P3348R4 C++26 Should Refer To C23, Not C17; but the issue only reverts a overlooked silent change
• LWG-4463, LWG-4464, and LWG-4465 patch P3179R9 Parallel Range Algorithms; but they look like pure wording fixes

[frederick-vs-ja]

It seems that we should implement LWG-4366 in C++23 mode. WG21-P2994R3 changed Mandates to Constraints but the operation shouldn't be changed when the comparison is already well-formed.

Currently we're returning static_cast<bool>(cmp-expr), while the resolution relies on implicit conversion to bool. The two approach can behave differently for some contrived types.

[StephanTLavavej]

Agreed, done! 😻