[PATCH v1] libstdc++: Make <mdspan> compatible with clang.

[Jonathan Wakely]

On Fri, 31 Oct 2025 at 12:44, Luc Grosheintz <luc.grosheintz@gmail.com> wrote:
>
>
> On 10/31/25 11:09, Jonathan Wakely wrote:
> > On Fri, 31 Oct 2025 at 08:23, Luc Grosheintz wrote:
> >>
> >> These three changes are needed to make <mdspan> compatible with Clang:
> >>
> >>    - the type alias _Storage must occur before its first use.
> >>
> >>    - the friend declarations of function must match exactly, including
> >>    noexcept and constexpr.
> >
> > Clang is correct here, GCC has a bug where it doesn't check the
> > declarations match.
> >
> >>
> >>    - the 'template' in typename T::template type<double>.
> >
> > I think Clang is correct here too.
> >
> > I'll apply this for you, thanks.
>
> Thank you! Please let me know if you'd like me to search bugzilla
> to check if these are already known (and report them on the off
> chance they're not).

The friend one is https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118381

I'm pretty sure we have an accepts-invalid bug for the template
keyword, but I didn't find it from a quick search.



>
> >
> >
> >>
> >> libstdc++-v3/ChangeLog:
> >>
> >>          * include/std/mdspan (extents::_Storage): Move type alias before
> >>          its first use.
> >>          (__mdspan::__static_extents): Add missing noexcept and constexpr
> >>          to friend declaration in extents.
> >>          (__mdspan::__dynamic_extents): Ditto.
> >>
> >> Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
> >> ---
> >>   libstdc++-v3/include/std/mdspan | 22 ++++++++++++----------
> >>   1 file changed, 12 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
> >> index d555b7f2580..bd7a2a201a7 100644
> >> --- a/libstdc++-v3/include/std/mdspan
> >> +++ b/libstdc++-v3/include/std/mdspan
> >> @@ -378,6 +378,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>         static_assert(
> >>            (__mdspan::__valid_static_extent<_Extents, _IndexType> && ...),
> >>            "Extents must either be dynamic or representable as IndexType");
> >> +
> >> +      using _Storage = __mdspan::_ExtentsStorage<
> >> +       _IndexType, array<size_t, sizeof...(_Extents)>{_Extents...}>;
> >> +      [[no_unique_address]] _Storage _M_exts;
> >> +
> >>       public:
> >>         using index_type = _IndexType;
> >>         using size_type = make_unsigned_t<index_type>;
> >> @@ -487,15 +492,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>          }
> >>
> >>       private:
> >> -      friend const array<size_t, rank()>&
> >> -      __mdspan::__static_extents<extents>();
> >> +      friend constexpr const array<size_t, rank()>&
> >> +      __mdspan::__static_extents<extents>() noexcept;
> >>
> >> -      friend span<const index_type>
> >> -      __mdspan::__dynamic_extents<extents>(const extents&, size_t, size_t);
> >> -
> >> -      using _Storage = __mdspan::_ExtentsStorage<
> >> -       _IndexType, array<size_t, sizeof...(_Extents)>{_Extents...}>;
> >> -      [[no_unique_address]] _Storage _M_exts;
> >> +      friend constexpr span<const index_type>
> >> +      __mdspan::__dynamic_extents<extents>(const extents&, size_t, size_t)
> >> +      noexcept;
> >>
> >>         template<typename _OIndexType, size_t... _OExtents>
> >>          friend class extents;
> >> @@ -1637,8 +1639,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> >>
> >>          template<typename _OExtents>
> >>            constexpr explicit
> >> -         _PaddedStorage(const typename _LayoutSame::mapping<_OExtents>&
> >> -                        __other)
> >> +         _PaddedStorage(
> >> +             const typename _LayoutSame::template mapping<_OExtents>& __other)
> >>            : _PaddedStorage(_Extents(__other.extents()))
> >>            {
> >>              constexpr size_t __stride_idx = _LayoutTraits::_S_stride_idx;
> >> --
> >> 2.51.1
> >>
> >
>