On Sat, 6 Sept 2025 at 14:12, Luc Grosheintz wrote:
>
> The concept __integral_constant_like doesn't consider traits with a
> boolean member `value` as an integer constant. This is done to reject
> various completely unrelated traits like is_const, is_abstract, etc.
>
> LWG4351 adjusts the check to strip references and cv qualifiers before
> checking if `value` is bool. The immediate context is constant_wrapper
> which defines:
>
> template<...>
> struct constant_wrapper
> {
> static constexpr const auto& value = ...;
> };
>
> Without LWG4351, std::cw<true> and std::cw<false> would both be
> considered integer constants (by __integral_constant_like); but both
> std::{true,false}_type are not considered integer constants. Hence,
> LWG4351 removes inconsistent behaviour between std::integral_constant
> and std::constant_wrapper.
Thanks, the patch looks good but I'd like to wait before pushing it
until LWG have seen the issue. I'll start a prioritization poll for
the issue tomorrow, and see if anybody points out a reason why the
issue is wrong and shouldn't be fixed (I don't expect that to happen
though).
>
> libstdc++-v3/ChangeLog:
>
> * include/std/span (__integral_constant_like): Use
> remove_cvref_t before checking if _Tp::value is boolean.
> * testsuite/23_containers/mdspan/extents/misc.cc: Update test.
> * testsuite/23_containers/mdspan/mdspan.cc: Ditto.
> * testsuite/23_containers/span/deduction.cc: Ditto.
>
> Signed-off-by: Luc Grosheintz <luc.groshei...@gmail.com>
> ---
> libstdc++-v3/include/std/span | 4 +++-
> libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc | 2 +-
> libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc | 3 +--
> libstdc++-v3/testsuite/23_containers/span/deduction.cc | 2 +-
> 4 files changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/span b/libstdc++-v3/include/std/span
> index f9aa3c77e8e..58089113565 100644
> --- a/libstdc++-v3/include/std/span
> +++ b/libstdc++-v3/include/std/span
> @@ -479,10 +479,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> // deduction guides
> namespace __detail
> {
> + // _GLIBCXX_RESOLVE_LIB_DEFECTS
> + // 4351. integral-constant-like needs more remove_cvref_t
> template<typename _Tp>
> concept __integral_constant_like =
> is_integral_v<remove_cvref_t<decltype(_Tp::value)>>
> - && !is_same_v<bool, remove_const_t<decltype(_Tp::value)>>
> + && !is_same_v<bool, remove_cvref_t<decltype(_Tp::value)>>
> && convertible_to<_Tp, decltype(_Tp::value)>
> && equality_comparable_with<_Tp, decltype(_Tp::value)>
> && bool_constant<_Tp() == _Tp::value>::value
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> index 94086653a74..33d6f947f0b 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/extents/misc.cc
> @@ -121,7 +121,7 @@ test_deduction_from_constant()
> verify(std::extents(1, c2), std::extents<size_t, dyn, 2>{1});
> verify(std::extents(c2), std::extents<size_t, 2>{});
> verify(std::extents(1, c2), std::extents<size_t, dyn, 2>{1});
> - verify(std::extents(std::cw<true>, c2), std::extents<size_t, 1, 2>{});
> + verify(std::extents(std::cw<true>, c2), std::extents<size_t, dyn, 2>{1});
> #endif
> return true;
> }
> diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> index ca100b4f314..a92a0554417 100644
> --- a/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> +++ b/libstdc++-v3/testsuite/23_containers/mdspan/mdspan.cc
> @@ -304,8 +304,7 @@ test_from_pointer_and_constant()
> auto c3 = std::constant_wrapper<3>{};
> verify(std::mdspan(ptr, 2, c3), std::extents(2, i3));
> verify(std::mdspan(ptr, 2, std::cw<3>), std::extents(2, i3));
> - verify(std::mdspan(ptr, std::cw<true>, std::cw<3>),
> - std::extents(std::cw<1>, i3));
> + verify(std::mdspan(ptr, std::cw<true>, std::cw<3>), std::extents(1, i3));
> #endif
> return true;
> }
> diff --git a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> index e958ad5d6db..55a586254e8 100644
> --- a/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> +++ b/libstdc++-v3/testsuite/23_containers/span/deduction.cc
> @@ -99,6 +99,6 @@ test01()
> static_assert( is_static_span<long, 4>(s17) );
>
> std::span s18(a.data(), std::cw<true>);
> - static_assert( is_static_span<long, 1>(s18) );
> + static_assert( is_dynamic_span<long>(s18) );
> #endif
> }
> --
> 2.51.0
>
