Meta LWG issue: 2023-02 meeting

[StephanTLavavej]

(Previous meta-issue: #3214)

At the February 2023 meeting, the following LWG issues were resolved in the C++ Working Paper.

❔ Not yet analyzed

• Remaining issues:

❌ 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-3441 Misleading note about calls to customization points
    • There's nothing to do here.
  • LWG-3827 Deprecate <stdalign.h> and <stdbool.h> macros
    • There's nothing for us to do here. These <meow.h> headers aren't part of the repo. Our <cstdalign> does define __alignas_is_defined and our <cstdbool> gets __bool_true_false_are_defined from <stdbool.h>, but the <cmeow> wrappers are already deprecated in C++17 mode (we didn't bother to implement removal in C++20 mode) and it's unclear how the UCRT could emit deprecation warnings for these macros. This isn't worth spending more time on, given that deprecation warnings aren't required for conformance (and we already go quite far here).
  • LWG-3828 Sync intmax_t and uintmax_t with C2x
  • LWG-3871 Adjust note about terminate
• Something that increases the restrictions placed on users, but implementers aren't expected to enforce those restrictions
  • LWG-3622 Misspecified transitivity of equivalence in [unord.req.general]
    • This is N/A because it increases restrictions on users without requiring implementers to enforce them.
  • LWG-3645 resize_and_overwrite is overspecified to call its callback with lvalues
    • There's nothing for implementers to do here (it merely increases the restrictions for users).
• Fixes for obviously broken wording, where implementers would have done the right thing anyways
  • LWG-3032 ValueSwappable requirement missing for push_heap and make_heap
  • LWG-3756 Is the std::atomic_flag class signal-safe?
  • LWG-3841 <version> should not be "all freestanding"
  • LWG-3843 std::expected<T, E>::value() & assumes E is copy constructible
    • This is "naturally enforced" (as we attempt to copy) and the error messages should be sufficiently clear.

😸 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-3085 char_traits::copy precondition too weak
  • LWG-3664 LWG-3392 broke std::ranges::distance(a, a + 3)
  • LWG-3723 priority_queue::push_range needs to append_range
  • LWG-3742 deque::prepend_range needs to permute
  • LWG-3807 The feature test macro for ranges::find_last should be renamed
  • LWG-3857 basic_string_view should allow explicit conversion when only traits vary
  • LWG-3867 Should std::basic_osyncstream's move assignment operator be noexcept?
    • Created PR Uncomment resolutions of applied LWG issues #3461 for all of these.
• Implemented without comments
  • LWG-3811 views::as_const on ref_view<T> should return ref_view<const T>
    • Noted as implemented by P2278R4: views::as_const #3234.
  • LWG-3825 Missing compile-time argument id check in basic_format_parse_context::next_arg_id
    • Implemented by <format>: compile time checks #2221.
  • LWG-3842 Unclear wording for precision in chrono-format-spec
    • Implemented:

      STL/stl/inc/chrono

      Lines 5301 to 5313 in 9ae1b3f

      	if constexpr (_Is_specialization_v<_Ty, duration>) {
      	if constexpr (!treat_as_floating_point_v<typename _Ty::rep>) {
      	if (_Specs._Precision != -1) {
      	_THROW(format_error("Precision specification invalid for chrono::duration type with "
      	"integral representation type, see N4885 [time.format]/1."));
      	}
      	}
      	} else {
      	if (_Specs._Precision != -1) {
      	_THROW(format_error("Precision specification invalid for non-chrono::duration type, "
      	"see N4885 [time.format]/1."));
      	}
      	}

    • We go beyond the Standard by using treat_as_floating_point_v. I think this is a remaining (small) defect in the Standardese.
    • When a new Working Paper is available, we could update the Standardese citation, but this is not absolutely necessary (as the stable name and paragraph number are unchanged, and the content of the paragraph hasn't changed that much).
  • LWG-3866 Bad Mandates for expected::transform_error overloads
    • Implemented by Implement P2505R5 Monadic Functions For std::expected #3361.

🩹 Patches an unimplemented feature

We should record 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.

• Recorded in P0533R9 constexpr For <cmath> And <cstdlib> #2530:
  • LWG-3834 Missing constexpr for std::intmax_t math functions in <cinttypes>
• Recorded in P2255R2 Type Traits To Detect References Binding To Temporaries #2534:
  • LWG-3819 reference_meows_from_temporary should not use is_meowible
• Recorded in P0009R18 <mdspan> #2909:
  • LWG-3876 Default constructor of std::layout_meow::mapping misses precondition
• Recorded in P0429R9 <flat_map> #2910 and P1222R4 <flat_set> #2912:
  • LWG-3786 Flat maps' deduction guides need to default Allocator to be useful
  • LWG-3803 flat_meow constructors taking KeyContainer lack KeyCompare parameter
• Recorded in P1222R4 <flat_set> #2912:
  • LWG-3879 erase_if for flat_{,multi}set is incorrectly specified
• Recorded in P2286R8 Formatting Ranges #2919:
  • LWG-3839 range_formatter's set_separator, set_brackets, and underlying functions should be noexcept
  • LWG-3881 Incorrect formatting of container adaptors backed by std::string
• Recorded in P2374R4 views::cartesian_product #2923:
  • LWG-3820 cartesian_product_view::iterator::prev is not quite right
  • LWG-3849 cartesian_product_view::iterator's default constructor is overconstrained
• Recorded in P1467R9 Extended Floating-Point Types #2956:
  • LWG-3790 WG21-P1467R9 accidentally changed nexttoward's signature

🐞 Not yet implemented

• Filed a GitHub issue labeled LWG
  • LWG-2195 Missing constructors for match_results
    • Filed LWG-2195 Missing constructors for match_results #3411.
  • LWG-2295 Locale name when the provided Facet is a nullptr
    • Filed LWG-2295 Locale name when the provided Facet is a nullptr #3463.
  • LWG-3204 sub_match::swap only swaps the base class
    • Filed LWG-3204 sub_match::swap only swaps the base class #3412.
  • LWG-3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout
    • Filed LWG-3631 basic_format_arg(T&&) should use remove_cvref_t<T> throughout #3460 and noted in P2286R8 Formatting Ranges #2919 (we implement some but not all of the machinery this issue touches).
  • LWG-3655 The INVOKE operation and union types
    • Filed LWG-3655 The INVOKE operation and union types #3424.
  • LWG-3720 Restrict the valid types of arg-id for width and precision in std-format-spec
    • Filed LWG-3720 Restrict the valid types of arg-id for width and precision in std-format-spec #3425.
  • LWG-3733 ranges::to misuses cpp17-input-iterator
    • Filed LWG-3733 ranges::to misuses cpp17-input-iterator #3422.
  • LWG-3734 Inconsistency in inout_ptr and out_ptr for empty case
    • Filed LWG-3734 Inconsistency in inout_ptr and out_ptr for empty case #3413.
  • LWG-3769 basic_const_iterator::operator== causes infinite constraint recursion
    • Filed LWG-3769 basic_const_iterator::operator== causes infinite constraint recursion #3458.
  • LWG-3821 uses_allocator_construction_args should have overload for pair-like
    • Filed LWG-3821 uses_allocator_construction_args should have overload for pair-like #3415. Should wait for Implement LWG-3677 Is a cv-qualified pair specially handled in uses-allocator construction? #3396 to be merged.
  • LWG-3833 Remove specialization template<size_t N> struct formatter<const charT[N], charT>
    • Filed LWG-3833 Remove specialization template<size_t N> struct formatter<const charT[N], charT> #3426.
  • LWG-3836 std::expected<bool, E1> conversion constructor expected(const expected<U, G>&) should take precedence over expected(U&&) with operator bool
    • Filed LWG-3836 std::expected<bool, E1> conversion constructor expected(const expected<U, G>&) should take precedence over expected(U&&) with operator bool #3433.
  • LWG-3847 ranges::to can still return views
    • Filed LWG-3847 ranges::to can still return views #3417.
  • LWG-3851 chunk_view::inner-iterator missing custom iter_move and iter_swap
    • Filed LWG-3851 chunk_view::inner-iterator missing custom iter_move and iter_swap #3432.
  • LWG-3853 basic_const_iterator<volatile int*>::operator-> is ill-formed
    • Filed LWG-3853 basic_const_iterator<volatile int*>::operator-> is ill-formed #3427.
  • LWG-3860 range_common_reference_t is missing
    • Filed LWG-3860 range_common_reference_t is missing #3416.
  • LWG-3862 basic_const_iterator's common_type specialization is underconstrained
    • Filed LWG-3862 basic_const_iterator's common_type specialization is underconstrained #3428.
  • LWG-3865 Sorting a range of pairs
    • Filed LWG-3865 Sorting a range of pairs #3419.
  • LWG-3869 Deprecate std::errc constants related to UNIX STREAMS
    • Filed LWG-3869 Deprecate std::errc constants related to UNIX STREAMS #3420.
  • LWG-3870 Remove voidify
    • Filed LWG-3870 Remove voidify #3431.
  • LWG-3872 basic_const_iterator should have custom iter_move
    • Filed LWG-3872 basic_const_iterator should have custom iter_move #3429.
  • LWG-3875 std::ranges::repeat_view<T, IntegerClass>::iterator may be ill-formed
    • Filed LWG-3875 std::ranges::repeat_view<T, IntegerClass>::iterator may be ill-formed #3430.
  • LWG-3877 Incorrect constraints on const-qualified monadic overloads for std::expected
    • Filed LWG-3877 Incorrect constraints on const-qualified monadic overloads for std::expected #3435.
  • LWG-3878 import std; should guarantee initialization of standard iostreams objects
    • Filed LWG-3878 import std; should guarantee initialization of standard iostreams objects #3436.
  • LWG-3880 Clarify operator+= complexity for {chunk,stride}_view::iterator
    • Filed LWG-3880 Clarify operator+= complexity for {chunk,stride}_view::iterator #3434.
• PR out for review
  • LWG-3772 repeat_view's piecewise constructor is missing preconditions
    • Implement an _STL_VERIFY check.
    • PR LWG-3772 repeat_view's piecewise constructor is missing Postconditions #3462.
  • LWG-3810 CTAD for std::basic_format_args
    • Filed LWG-3810 CTAD for std::basic_format_args #3414.
    • PR LWG-3810: CTAD for basic_format_args #3421.
  • LWG-3848 adjacent_view, adjacent_transform_view, and slide_view missing base accessor
    • Filed LWG-3848 adjacent_view, adjacent_transform_view, and slide_view missing base accessor #3409.
    • PR LWG-3848: slide_view missing base accessor #3410.
    • Also recorded in P2321R2 zip #2252.
  • LWG-3850 views::as_const on empty_view<T> should return empty_view<const T>
    • Filed LWG-3850 views::as_const on empty_view<T> should return empty_view<const T> #3418.
    • PR LWG-3850: views::as_const on empty_view<T> should return empty_view<const T> #3423.

[CaseyCarter]

Confirmed: LWG-3441 is definitely N/A. The resolution simply strikes the confusing "designated customization point" wording that LWG thinks contributed only confusion.
Confirmed: LWG-3662 is N/A as well. The constraints on program-defined transparent equality comparators were insufficient, this makes them stronger.
Confirmed: LWG-3645 is N/A. User function objects must accept const-or-not lvalue-or-rvalue arguments - so implementations can pass whatever they like.

[CaseyCarter]

• LWG-3843 std::expected<T, E>::value() & assumes E is copy constructible
  • It looks like this is "naturally enforced" (as we attempt to copy), but we might want to add static_asserts.

I'm happy not to static_assert; I think the resulting error messages about being unable to copy will be clear enough.

[frederick-vs-ja]

• This is "naturally enforced" (as we attempt to copy) and the error messages should be sufficiently clear.

This is currently not enforced by MSVC (DevCom-1638273). I'll open a PR for workaround.