[gcc r16-5421] libstdc++: Make <mdspan> compatible with clang.

[Tomasz Kaminski]

https://gcc.gnu.org/g:58b88748e6bf651d425ae11d143d02a29575186b

commit r16-5421-g58b88748e6bf651d425ae11d143d02a29575186b
Author: Luc Grosheintz <luc.grosheintz@gmail.com>
Date:   Wed Nov 19 14:52:04 2025 +0100

    libstdc++: Make <mdspan> compatible with clang.
    
    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.
    
      - the 'template' in typename T::template type<double>.
    
    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.
    
    Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
    Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>

Diff:
---
 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 d555b7f2580a..bd7a2a201a74 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;