[PATCH v1] libstdc++: Apply LWG4351 to CTAD of span/mdspan.

[Jonathan Wakely]

On Sun, 7 Sept 2025 at 16:34, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> 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).

Unsurprisingly, LWG agreed with the proposed resolution. So this is OK
fo trunk now.


>
> >
> > 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.grosheintz@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
> >