Meta LWG issue: 2024-11 meeting

[StephanTLavavej]

(Previous meta-issue: #4759)

At the November 2024 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. Categories:

• 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-4064 Clarify that std::launder is not needed when using the result of std::memcpy
  • LWG-4126 Some feature-test macros for fully freestanding features are not yet marked freestanding
  • LWG-4141 Improve prohibitions on "additional storage"
• Something that increases the restrictions placed on users, but implementers aren't expected to enforce those restrictions
  • LWG-4170 contiguous_iterator should require to_address(I{})
• Fixes for obviously broken wording, where implementers would have done the right thing anyways

😸 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 - e.g. 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-4024 Underspecified destruction of objects created in std::make_shared_for_overwrite/std::allocate_shared_for_overwrite
    • Implemented by <memory>: Add different control block types for allocate_shared_for_overwrite #4274, comment removal tracked by Remove comments for LWG-3210, LWG-4024, and LWG-4154 #5127
  • LWG-4154 The Mandates for std::packaged_task's constructor from a callable entity should consider decaying
    • Implemented by <future>: Make packaged_task accept move-only functors #4946, comment removal tracked by Remove comments for LWG-3210, LWG-4024, and LWG-4154 #5127
  • LWG-3216 Rebinding the allocator before calling construct/destroy in allocate_shared
    • This also resolved LWG-3210 "allocate_shared is inconsistent about removing const from the pointer passed to allocator construct and destroy", which we've implemented since 2019. Comment removal tracked by Remove comments for LWG-3210, LWG-4024, and LWG-4154 #5127
• Implemented without comments
  • LWG-4157 The resolution of LWG-3465 was damaged by WG21-P2167R3 (in compare_partial_order_fallback)
    • Implemented by Implement P2167R3 Improving boolean-testable Usage (for comparison fallback CPOs) #3258
  • LWG-4088 println ignores the locale imbued in std::ostream
    • Implemented by P2093R14: Formatted Output #3337

🩹 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.
  • LWG-4085 ranges::generate_random's helper lambda should specify the return type
    • Recorded in P1068R11 ranges::generate_random #4512
  • LWG-4147 Precondition on inplace_vector::emplace
    • Recorded in P0843R14 <inplace_vector> #4766
  • LWG-4134 Issue with Philox algorithm specification
    • Recorded in P2075R6 philox_engine #4767
  • LWG-4153 Fix extra "-1" for philox_engine::max()
    • Recorded in P2075R6 philox_engine #4767
  • LWG-4142 format_parse_context::check_dynamic_spec should require at least one type
    • Recorded in P2757R3 Type-Checking Format Args #3813
  • LWG-4148 unique_ptr::operator* should not allow dangling references
    • Recorded in P2255R2 Type Traits To Detect References Binding To Temporaries #2534

🐞 Not yet implemented

• Filed a GitHub issue labeled LWG
  • LWG-3436 std::construct_at should support arrays
    • LWG-3436 std::construct_at should support arrays #5110
  • LWG-3899 co_yielding elements of an lvalue generator is unnecessarily inefficient
    • LWG-3899 co_yielding elements of an lvalue generator is unnecessarily inefficient #5111
  • LWG-3900 The allocator_arg_t overloads of generator::promise_type::operator new should not be constrained
    • LWG-3900 The allocator_arg_t overloads of generator::promise_type::operator new should not be constrained #5112
  • LWG-3918 std::uninitialized_move/_n and guaranteed copy elision
    • LWG-3918 std::uninitialized_move/_n and guaranteed copy elision #5113
  • LWG-4027 possibly-const-range should prefer returning const R&
    • LWG-4027 possibly-const-range should prefer returning const R& #5114
  • LWG-4072 std::optional comparisons: constrain harder
    • LWG-4072 std::optional comparisons: constrain harder #5115
  • LWG-4112 has-arrow should require operator->() to be const-qualified
    • LWG-4112 has-arrow should require operator->() to be const-qualified #5116
  • LWG-4135 The helper lambda of std::erase for list should specify return type as bool
    • LWG-4135 The helper lambda of std::erase for list should specify return type as bool #5117
  • LWG-4119 generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed
    • LWG-4119 generator::promise_type::yield_value(ranges::elements_of<R, Alloc>)'s nested generator may be ill-formed #5118
  • LWG-3886 Monad mo' problems (in optional and expected)
    • LWG-3886 Monad mo' problems (in optional and expected) #5119
  • LWG-4084 std::fixed ignores std::uppercase
    • LWG-4084 std::fixed ignores std::uppercase #5120
  • LWG-4124 Cannot format zoned_time with resolution coarser than seconds
    • LWG-4124 Cannot format zoned_time with resolution coarser than seconds #5121
  • LWG-4140 Useless default constructors for bit reference types
    • LWG-4140 Useless default constructors for bit reference types #5122
  • LWG-4169 std::atomic<T>'s default constructor should be constrained
    • LWG-4169 std::atomic<T>'s default constructor should be constrained #5123
  • LWG-4113 Disallow has_unique_object_representations<Incomplete[]>
    • LWG-4113 Disallow has_unique_object_representations<Incomplete[]> #5124
  • LWG-4144 Disallow unique_ptr<T&, D>
    • LWG-4144 Disallow unique_ptr<T&, D> #5125
  • LWG-4164 Missing guarantees for forward_list modifiers
    • LWG-4164 Missing guarantees for forward_list modifiers #5126
  • LWG-4014 LWG-3809 changes behavior of some existing std::subtract_with_carry_engine code
    • LWG-4014 LWG-3809 changes behavior of some existing std::subtract_with_carry_engine code #5133
  • LWG-4044 Confusing requirements for std::print on POSIX platforms
    • No action required for correctness on Windows, but filed performance followup <print>: Investigate optimizing for UTF-8 terminals #5134
• PR out for review

[frederick-vs-ja]

LWG-4024 is implemented by #4274. Comments should be removed.
LWG-4064 is probably N/A for library implementations. It should be handled by compiler frontends.
LWG-4126 is N/A for out hosted implementation. We always provide these feature-macros in corresponding modes.
LWG-4135 is implemented by #4900. No comment added.
LWG-4141 is N/A or already implemented. The issue mainly makes the intent of designs more explicit.
LWG-4154 is implemented by #4946. Comments should be removed.
LWG-4157 is implemented by #3258. No comment added.

[StephanTLavavej]

Thanks! I've applied all of that, except that I filed #5117 because I noticed that it's not fully implemented.

[CaseyCarter]

LWG-4044 Confusing requirements for std::print on POSIX platforms

• @StephanTLavavej thinks this should have no effect for Windows; need to double-check

There exist terminals that can only display Unicode via a "native Unicode API", and terminals that can display Unicode simply via the normal means of dumping output. The intent of the issue is that only the former require the determination if a stream refers to such a terminal, and require flushing the "normal" output before invoking such a "native Unicode API" to output Unicode.

In real world terms, "native Unicode API" means "WriteConsoleW on Windows". Non-Windows platforms are supposed to ignore these instructions but it wasn't clear to them that they should, so the issue is trying to clarify that intent.

Of course there are also terminals on Windows that are capable of displaying Unicode via the normal output mechanisms. My machine with the "Beta: Use UTF-8 for language support" feature activated defaults to codepage 65001 in terminals and doesn't need "native Unicode API" to output Unicode. At some point we should start detecting this condition and avoiding the mess of flushing stdout and transcoding to UTF-16 to call WriteConsoleW.

TLDR: We don't need to do anything for correctness with regard to LWG-4044, we may want to make changes for performance.

[frederick-vs-ja]

[ ]
LWG-4014 LWG-3809 changes behavior of some existing std::subtract_with_carry_engine code

• Stephan T. Lavavej (@StephanTLavavej) glanced at this and it appears to be already implemented; need to double-check

This hasn't been implemented. It was noted in #4194 (comment) that the resolution of LWG-3809 is behavior-changing (some changes in the test file also proved this). LWG-4014 restores the old behavior.