From: Luc Grosheintz <luc.grosheintz@gmail.com>
Date: Mon, 8 Dec 2025 20:23:41 +0000 (+0100)
Subject: libstdc++: Implement submdspan and submdspan_mapping for layout_left. [PR110352]
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ead579d3c52a3b1d8ba5f337a2c7c3bd3749c011;p=thirdparty%2Fgcc.git

libstdc++: Implement submdspan and submdspan_mapping for layout_left. [PR110352]

Implements `submdspan` and `submdspan_mapping` for layout_left as
described in P3663 (Future proofing mdspan).

When computing the offset of the submdspan, one must check that the
lower bound of the slice range isn't out-of-range. There's a few
cases when the lower bound is never out-of-range:

  - full_extent and exts.extent(k) != 0,
  - collapsing slice types.

If those conditions are known to hold, no checks are generated.

Similarly, if all slices are full_extent, there's no need to call
mapping(0,...,0) for standardized mappings.

The implementation prepares to use the symmetry between layout_left and
layout_right and introduces concepts like a "layout side", i.e. left,
right or unknown/strided.

The tests use an iterator to replace nested for-loops. Which also makes
it easier to write the core test logic in a rank-independent manner.

	PR libstdc++/110352

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__is_submdspan_mapping_result)
	(__mdspan::__submdspan_mapping_result, __mdspan::__fwd_prod)
	(__mdspan::__acceptable_slice_type, __mdspan::__slice_begin)
	(__mdspan::__suboffset, __mdspan::_LayoutSide, __mdspan::__mapping_side)
	(__mdspan::_StridesTrait, __mdspan::__substrides_generic)
	(__mdspan::__substrides_standardized, __mdspan::__substrides)
	(__mdspan::__is_unit_stride_slice, __mdspan::_SliceKind)
	(__mdspan::__make_slice_kind, __mdspan::__make_slice_kind_array)
	(__mdspan::__is_block, __mdspan::__padded_block_begin_generic)
	(__mdspan::__padded_block_begin, __mpdspan::_SubMdspanMapping)
	(__mdspan::__submdspan_mapping_impl): Define.
	(__mdspan::__dynamic_slice_extent, __mdspan::__static_slice_extent)
	(__mdspan::__subextents): Move earlier in the file.
	(layout_left::mapping::submdspan_mapping, __mdspan::__sliceable_mapping)
	(__mdspan::__submapping, submdspan): Define.
	* src/c++23/std.cc.in: Add submdspan.
	* testsuite/23_containers/mdspan/submdspan/generic.cc: New test.
	* testsuite/23_containers/mdspan/submdspan/selections/left.cc:
	Instantiate selection tests for layout_left.
	* testsuite/23_containers/mdspan/submdspan/selections/testcases.h: Generic
	tests different selections.
	* testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc: New test.
	* testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc: New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>
---

diff --git a/libstdc++-v3/include/std/mdspan b/libstdc++-v3/include/std/mdspan
index f0f6630b472..0d477b7fa9e 100644
--- a/libstdc++-v3/include/std/mdspan
+++ b/libstdc++-v3/include/std/mdspan
@@ -374,6 +374,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       [[no_unique_address]] _Mapping mapping = _Mapping();
       size_t offset{};
     };
+
+  template<typename _Tp>
+    constexpr bool __is_submdspan_mapping_result = false;
+
+  template<typename _Mapping>
+    constexpr bool __is_submdspan_mapping_result<submdspan_mapping_result<_Mapping>> = true;
+
+  template<typename _Mapping>
+    concept __submdspan_mapping_result = __is_submdspan_mapping_result<_Mapping>;
+
 #endif // __glibcxx_submdspan
 
   template<typename _IndexType, size_t... _Extents>
@@ -589,6 +599,16 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  }
       }
 
+    template<typename _IndexType, size_t _Nm>
+      consteval _IndexType
+      __fwd_prod(span<const _IndexType, _Nm> __values)
+      {
+	_IndexType __ret = 1;
+	for(auto __value : __values)
+	  __ret *= __value;
+	return __ret;
+      }
+
     // Preconditions: _r < _Extents::rank()
     template<typename _Extents>
       constexpr typename _Extents::index_type
@@ -866,6 +886,11 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __exts.extent(_Index)};
       }
 
+    template<typename _Slice, typename _IndexType>
+      concept __acceptable_slice_type = same_as<_Slice, full_extent_t>
+	  || same_as<_Slice, _IndexType> || __is_constant_wrapper<_Slice>
+	  || __is_strided_slice<_Slice>;
+
     template<typename _IndexType, typename... _Slices>
       consteval auto
       __subrank()
@@ -890,6 +915,443 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    __map[__i++] = __k;
 	return __map;
       }
+
+    template<typename _Slice>
+      constexpr auto
+      __slice_begin(_Slice __slice)
+      {
+	if constexpr (same_as<_Slice, full_extent_t>)
+	  return 0;
+	else if constexpr (__is_strided_slice<_Slice>)
+	  return __slice.offset;
+	else
+	  return __slice; // collapsing slice
+      }
+
+    template<typename _Mapping, typename... _Slices>
+      constexpr size_t
+      __suboffset(const _Mapping& __mapping, const _Slices&... __slices)
+      {
+	using _IndexType = typename _Mapping::index_type;
+	auto __any_past_the_end = [&]<size_t... _Is>(index_sequence<_Is...>)
+	{
+	  auto __is_past_the_end = [](const auto& __slice, const auto& __ext)
+	  {
+	    using _Slice = remove_cvref_t<decltype(__slice)>;
+	    if constexpr (is_convertible_v<_Slice, _IndexType>)
+	      return false;
+	    else if constexpr (same_as<_Slice, full_extent_t>
+		&& __ext.static_extent(0) != 0
+		&& __ext.static_extent(0) != dynamic_extent)
+	      return false;
+	    else
+	      return __mdspan::__slice_begin(__slice) == __ext.extent(0);
+	  };
+
+	  const auto& __exts = __mapping.extents();
+	  return ((__is_past_the_end(__slices...[_Is],
+				     __mdspan::__extract_extent<_Is>(__exts))) || ...);
+	};
+
+	if constexpr ((same_as<_Slices, full_extent_t> && ...))
+	  return __mdspan::__offset(__mapping);
+
+	if (__any_past_the_end(std::make_index_sequence<sizeof...(__slices)>()))
+	  return __mapping.required_span_size();
+	return __mapping(__mdspan::__slice_begin(__slices)...);
+      }
+
+    template<typename _IndexType, size_t _Extent, typename _Slice>
+      consteval size_t
+      __static_slice_extent()
+      {
+	if constexpr (same_as<_Slice, full_extent_t>)
+	  return _Extent;
+	else if constexpr (same_as<_Slice, constant_wrapper<_IndexType(0)>>)
+	  return 0;
+	else if constexpr (__is_constant_wrapper<typename _Slice::extent_type>
+			&& __is_constant_wrapper<typename _Slice::stride_type>)
+	  return 1 + ((typename _Slice::extent_type{}) - 1)
+		   / (typename _Slice::stride_type{});
+	else
+	  return dynamic_extent;
+      }
+
+    template<size_t _K, typename _Extents, typename _Slice>
+      constexpr typename _Extents::index_type
+      __dynamic_slice_extent(const _Extents& __exts, _Slice __slice)
+      {
+	if constexpr (__is_strided_slice<_Slice>)
+	  return __slice.extent == 0 ? 0 : 1 + (__slice.extent - 1) / __slice.stride;
+	else
+	  return __exts.extent(_K);
+      }
+
+    template<typename _IndexType, size_t... _Extents, typename... _Slices>
+      requires (sizeof...(_Slices) == sizeof...(_Extents))
+      constexpr auto
+      __subextents(const extents<_IndexType, _Extents...>& __exts,
+		   _Slices... __slices)
+      {
+	constexpr auto __inv_map = __mdspan::__inv_map_rank<_IndexType, _Slices...>();
+	auto __impl = [&]<size_t... _Indices>(std::index_sequence<_Indices...>)
+	{
+	  using _SubExts = extents<_IndexType,
+	      __mdspan::__static_slice_extent<_IndexType,
+		 _Extents...[__inv_map[_Indices]],
+		 _Slices...[__inv_map[_Indices]]>()...>;
+	  if constexpr (_SubExts::rank_dynamic() == 0)
+	    return _SubExts{};
+	  else
+	    {
+	      using _StaticSubExtents = __mdspan::_StaticExtents<
+		__mdspan::__static_extents<_SubExts>()>;
+	      auto __create = [&]<size_t... _Is>(std::index_sequence<_Is...>)
+	      {
+		constexpr auto __slice_idx = [__inv_map](size_t __i) consteval
+		{
+		  return __inv_map[_StaticSubExtents::_S_dynamic_index_inv(__i)];
+		};
+
+		return _SubExts{__mdspan::__dynamic_slice_extent<__slice_idx(_Is)>(
+		     __exts, __slices...[__slice_idx(_Is)])...};
+	      };
+	      constexpr auto __dyn_subrank = _SubExts::rank_dynamic();
+	      return __create(std::make_index_sequence<__dyn_subrank>());
+	    }
+	};
+
+	return __impl(std::make_index_sequence<__inv_map.size()>());
+      }
+
+    enum class _LayoutSide
+    {
+      __left,
+      __right,
+      __unknown
+    };
+
+    template<typename _Mapping>
+      consteval _LayoutSide
+      __mapping_side()
+      {
+	if constexpr (__is_left_padded_mapping<_Mapping>
+	    || __mapping_of<layout_left, _Mapping>)
+	  return _LayoutSide::__left;
+	if constexpr (__is_right_padded_mapping<_Mapping>
+	    || __mapping_of<layout_right, _Mapping>)
+	  return _LayoutSide::__right;
+	else
+	  return _LayoutSide::__unknown;
+      }
+
+    template<_LayoutSide _Side, size_t _Rank>
+      struct _StridesTrait
+      {
+	static constexpr const _LayoutSide _S_side = _Side;
+
+	static constexpr size_t
+	_S_idx(size_t __k) noexcept
+	{
+	  if constexpr (_Side == _LayoutSide::__left)
+	    return __k;
+	  else
+	    return _Rank - 1 - __k;
+	}
+
+	// Unifies the formulas for computing strides for padded and unpadded
+	// layouts.
+	template<typename _Mapping>
+	  static constexpr typename _Mapping::index_type
+	  _S_padded_extent(const _Mapping& __mapping, size_t __k)
+	  {
+	    if (__k == 0)
+	      return __mapping.stride(_S_idx(1));
+	    else
+	      return __mapping.extents().extent(_S_idx(__k));
+	  }
+
+	template<typename _IndexType, typename... _Slices>
+	  static consteval auto
+	  _S_inv_map()
+	  {
+	    static_assert(_Side != _LayoutSide::__unknown);
+	    auto __impl = [&]<size_t... _Is>(std::index_sequence<_Is...>)
+	    {
+	      return __mdspan::__inv_map_rank<_IndexType, _Slices...[_S_idx(_Is)]...>();
+	    };
+	    return __impl(std::make_index_sequence<_Rank>());
+	  }
+      };
+
+    template<typename _SubExts, typename _Mapping, typename... _Slices>
+      constexpr auto
+      __substrides_generic(const _Mapping& __mapping, const _Slices&... __slices)
+      {
+	using _IndexType = typename _Mapping::index_type;
+	if constexpr (_SubExts::rank() == 0)
+	  return array<_IndexType, _SubExts::rank()>{};
+	else
+	  {
+	    auto __stride = [&__mapping](size_t __k, auto __slice) -> _IndexType
+	    {
+	      if constexpr (__is_strided_slice<decltype(__slice)>)
+		if (__slice.stride < __slice.extent)
+		  return __mapping.stride(__k) * __slice.stride;
+	      return __mapping.stride(__k);
+	    };
+
+	    auto __impl = [&]<size_t... _Is>(std::index_sequence<_Is...>)
+	    {
+	      constexpr auto __inv_map
+		= __mdspan::__inv_map_rank<_IndexType, _Slices...>();
+	      return array<_IndexType, _SubExts::rank()>{
+		__stride(__inv_map[_Is], __slices...[__inv_map[_Is]])...};
+	    };
+	    return __impl(std::make_index_sequence<_SubExts::rank()>());
+	  }
+      };
+
+    template<typename _SubExts, typename _Mapping, typename... _Slices>
+      constexpr auto
+      __substrides_standardized(const _Mapping& __mapping,
+			       const _Slices&... __slices)
+      {
+	using _IndexType = typename _Mapping::index_type;
+	using _Trait = _StridesTrait<__mapping_side<_Mapping>(),
+				     _Mapping::extents_type::rank()>;
+	using _SubTrait = _StridesTrait<__mapping_side<_Mapping>(), _SubExts::rank()>;
+
+	constexpr size_t __sub_rank = _SubExts::rank();
+
+	std::array<_IndexType, __sub_rank> __ret;
+	if constexpr (__sub_rank > 0)
+	  {
+	    constexpr auto __inv_map
+	      = _Trait::template _S_inv_map<_IndexType, _Slices...>();
+	    auto __loop = [&]<size_t... _Ks>(std::index_sequence<_Ks...>)
+	    {
+	      size_t __i0 = 0;
+	      size_t __stride = 1;
+	      auto __body = [&](size_t __k, auto __slice)
+	      {
+		for (size_t __i = __i0; __i < __inv_map[__k]; ++__i)
+		  __stride *= _Trait::_S_padded_extent(__mapping, __i);
+
+		size_t __krev = _SubTrait::_S_idx(__k);
+		if constexpr (__is_strided_slice<decltype(__slice)>)
+		  {
+		    if (__slice.stride < __slice.extent)
+		      __ret[__krev] = __stride * __slice.stride;
+		    else
+		      __ret[__krev] = __stride;
+		  }
+		else
+		  __ret[__krev] = __stride;
+
+		__i0 = __inv_map[__k];
+	      };
+
+	      ((__body(_Ks, __slices...[_Trait::_S_idx(__inv_map[_Ks])])),...);
+	    };
+	    __loop(std::make_index_sequence<__sub_rank>());
+	  }
+	return __ret;
+      }
+
+
+    template<typename _SubExts, typename _Mapping, typename... _Slices>
+      constexpr auto
+      __substrides(const _Mapping& __mapping, const _Slices&... __slices)
+      {
+	if constexpr (__mdspan::__mapping_side<_Mapping>() == _LayoutSide::__unknown)
+	  return __mdspan::__substrides_generic<_SubExts>(__mapping, __slices...);
+	else
+	  return __mdspan::__substrides_standardized<_SubExts>(__mapping, __slices...);
+      }
+
+    template<typename _Slice>
+      concept __is_unit_stride_slice = (__mdspan::__is_strided_slice<_Slice>
+	  && __mdspan::__is_constant_wrapper<typename _Slice::stride_type>
+	  && _Slice::stride_type::value == 1)
+	|| std::same_as<_Slice, full_extent_t>;
+
+    // These are (forced) exclusive categories:
+    //  - full & collapsing: obvious,
+    //  - unit_strided_slice: strided_slice{a, b, cw<1>}, but not `full`,
+    //  - strided_slice: strided_slice{a, b, c} with c != cw<1>.
+    enum class _SliceKind
+    {
+      __strided_slice,
+      __unit_strided_slice,
+      __full,
+      __collapsing
+    };
+
+    template<typename _Slice>
+      consteval _SliceKind
+      __make_slice_kind()
+      {
+	if constexpr (std::same_as<_Slice, full_extent_t>)
+	  return _SliceKind::__full;
+	else if constexpr (__mdspan::__is_strided_slice<_Slice>)
+	  {
+	    if constexpr (__mdspan::__is_unit_stride_slice<_Slice>)
+	      return _SliceKind::__unit_strided_slice;
+	    else
+	      return _SliceKind::__strided_slice;
+	  }
+	else
+	  return _SliceKind::__collapsing;
+      }
+
+    template<typename... _Slices>
+      consteval array<_SliceKind, sizeof...(_Slices)>
+      __make_slice_kind_array()
+      {
+	return array<_SliceKind, sizeof...(_Slices)>{
+	  __mdspan::__make_slice_kind<_Slices>()...};
+      }
+
+    //                   __block_size - 1
+    // [full, ..., full, unit_slice    , *]
+    consteval bool
+    __is_block(span<const _SliceKind> __slice_kinds, size_t __block_size)
+    {
+      if (__block_size == 0)
+	return false;
+
+      if (__block_size > __slice_kinds.size())
+	return false;
+
+      for (size_t __i = 0; __i < __block_size - 1; ++__i)
+	if (__slice_kinds[__i] != _SliceKind::__full)
+	  return false;
+
+      auto __last = __slice_kinds[__block_size - 1];
+      return __last == _SliceKind::__full
+	  || __last == _SliceKind::__unit_strided_slice;
+    }
+
+    //                         __u              __u + __sub_rank-2
+    // [unit_slice, i, ..., k, full, ..., full, unit_slice, *]
+    static consteval size_t
+    __padded_block_begin_generic(span<const _SliceKind> __slice_kinds,
+				 size_t __sub_rank)
+    {
+      if (__slice_kinds[0] != _SliceKind::__full
+	  && __slice_kinds[0] != _SliceKind::__unit_strided_slice)
+	return dynamic_extent;
+      else if (__slice_kinds.size() == 1)
+	return dynamic_extent;
+      else
+	{
+	  size_t __u = 1;
+	  while(__u < __slice_kinds.size()
+		&& __slice_kinds[__u] == _SliceKind::__collapsing)
+	    ++__u;
+
+	  if (__mdspan::__is_block(__slice_kinds.subspan(__u), __sub_rank -1))
+	    return __u;
+	  return dynamic_extent;
+	}
+    }
+
+    template<_LayoutSide _Side, size_t _Nm>
+      static consteval size_t
+      __padded_block_begin(span<const _SliceKind, _Nm> __slice_kinds, size_t __sub_rank)
+      {
+	if constexpr (_Side == _LayoutSide::__left)
+	  return __mdspan::__padded_block_begin_generic(__slice_kinds, __sub_rank);
+	else
+	  {
+	    std::array<_SliceKind, _Nm> __rev_slices;
+	    for(size_t __i = 0; __i < _Nm; ++__i)
+	      __rev_slices[__i] = __slice_kinds[_Nm - 1 - __i];
+	    auto __rev_slice_kinds = span<const _SliceKind>(__rev_slices);
+
+	    auto __u = __mdspan::__padded_block_begin_generic(__rev_slice_kinds,
+							      __sub_rank);
+	    return __u == dynamic_extent ? dynamic_extent : _Nm - 1 - __u;
+	  }
+      }
+
+    template<_LayoutSide _Side>
+      struct _SubMdspanMapping;
+
+    template<>
+      struct _SubMdspanMapping<_LayoutSide::__left>
+      {
+	using _Layout = layout_left;
+	template<size_t _Pad> using _PaddedLayout = layout_left_padded<_Pad>;
+
+	template<typename _Mapping, size_t _Us>
+	  static consteval size_t
+	  _S_pad()
+	  {
+	    using _Extents = typename _Mapping::extents_type;
+	    constexpr auto __sta_exts = __mdspan::__static_extents<_Extents>(0, _Us);
+	    if constexpr (!__mdspan::__all_static(__sta_exts))
+	      return dynamic_extent;
+	    else
+	      return __mdspan::__fwd_prod(__sta_exts);
+	  }
+
+	template<size_t _Nm>
+	  static consteval bool
+	  _S_is_unpadded_submdspan(span<const _SliceKind, _Nm> __slice_kinds, size_t __sub_rank)
+	  { return __mdspan::__is_block(__slice_kinds, __sub_rank); }
+      };
+
+    template<typename _Mapping>
+      constexpr auto
+      __submdspan_mapping_impl(const _Mapping& __mapping)
+      { return submdspan_mapping_result{__mapping, 0}; }
+
+    template<typename _Mapping, typename... _Slices>
+      requires (sizeof...(_Slices) > 0)
+      constexpr auto
+      __submdspan_mapping_impl(const _Mapping& __mapping, _Slices... __slices)
+      {
+	using _IndexType = typename _Mapping::index_type;
+	static_assert((__acceptable_slice_type<_Slices, _IndexType> && ...));
+
+	constexpr auto __side = __mdspan::__mapping_side<_Mapping>();
+	constexpr auto __rank = sizeof...(_Slices);
+	using _Trait = _SubMdspanMapping<__side>;
+	using _SliceView = span<const _SliceKind, __rank>;
+
+	constexpr auto __slice_kinds = __mdspan::__make_slice_kind_array<_Slices...>();
+	auto __offset = __mdspan::__suboffset(__mapping, __slices...);
+	auto __sub_exts = __mdspan::__subextents(__mapping.extents(), __slices...);
+	using _SubExts = decltype(__sub_exts);
+	constexpr auto __sub_rank = _SubExts::rank();
+	if constexpr (__sub_rank == 0)
+	  return submdspan_mapping_result{
+	    typename _Trait::_Layout::mapping(__sub_exts), __offset};
+	else if constexpr (_Trait::_S_is_unpadded_submdspan(
+	      _SliceView(__slice_kinds), __sub_rank))
+	  return submdspan_mapping_result{
+	    typename _Trait::_Layout::mapping(__sub_exts), __offset};
+	else if constexpr (
+	    constexpr auto __u = __padded_block_begin<__side>(
+		_SliceView(__slice_kinds), __sub_rank);
+	    __u != dynamic_extent)
+	  {
+	    constexpr auto __pad = _Trait::template _S_pad<_Mapping, __u>();
+	    using _Layout = typename _Trait::template _PaddedLayout<__pad>;
+	    return submdspan_mapping_result{
+	      typename _Layout::mapping(__sub_exts, __mapping.stride(__u)),
+	      __offset};
+	  }
+	else
+	  {
+	    auto __sub_strides
+	      = __mdspan::__substrides<_SubExts>(__mapping, __slices...);
+	    return submdspan_mapping_result{
+	      layout_stride::mapping(__sub_exts, __sub_strides),  __offset};
+	  }
+      }
 #endif // __glibcxx_submdspan
   }
 
@@ -1032,6 +1494,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	  __glibcxx_assert(__mdspan::__is_representable_extents(_M_extents));
 	}
 
+#if __glibcxx_submdspan
+      template<typename... _Slices>
+	requires (extents_type::rank() == sizeof...(_Slices))
+	friend constexpr auto
+	submdspan_mapping(const mapping& __mapping, _Slices... __slices)
+	{ return __mdspan::__submdspan_mapping_impl(__mapping, __slices...); }
+#endif // __glibcxx_submdspan
+
 	[[no_unique_address]] extents_type _M_extents{};
     };
 
@@ -2700,68 +3170,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	__impl(make_index_sequence<__rank>());
       }
 
-    template<typename _IndexType, size_t _Extent, typename _Slice>
-      consteval size_t
-      __static_slice_extent()
-      {
-	if constexpr (same_as<_Slice, full_extent_t>)
-	  return _Extent;
-	else if constexpr (same_as<_Slice, constant_wrapper<_IndexType(0)>>)
-	  return 0;
-	else if constexpr (__is_constant_wrapper<typename _Slice::extent_type>
-			&& __is_constant_wrapper<typename _Slice::stride_type>)
-	  return 1 + ((typename _Slice::extent_type{}) - 1)
-		   / (typename _Slice::stride_type{});
-	else
-	  return dynamic_extent;
-      }
+    template<typename _Slice>
+      using __full_extent_t = std::full_extent_t;
 
-    template<size_t _K, typename _Extents, typename _Slice>
-      constexpr typename _Extents::index_type
-      __dynamic_slice_extent(const _Extents& __exts, _Slice __slice)
+    // Enables ADL-only calls from submdspan.
+    void submdspan_mapping() = delete;
+
+    template<typename _Mapping, typename... _Slices>
+      concept __sliceable_mapping = requires(const _Mapping __m, _Slices... __slices)
       {
-	if constexpr (__is_strided_slice<_Slice>)
-	  return __slice.extent == 0 ? 0 : 1 + (__slice.extent - 1) / __slice.stride;
-	else
-	  return __exts.extent(_K);
-      }
+	{ submdspan_mapping(__m, __slices...) } -> __submdspan_mapping_result;
+      };
 
-    template<typename _IndexType, size_t... _Extents, typename... _Slices>
-      requires (sizeof...(_Slices) == sizeof...(_Extents))
+    template<typename _Mapping, typename... _Slices>
       constexpr auto
-      __subextents(const extents<_IndexType, _Extents...>& __exts,
-		   _Slices... __slices)
+      __submapping(const _Mapping& __mapping, _Slices... __slices)
       {
-	constexpr auto __inv_map = __mdspan::__inv_map_rank<_IndexType, _Slices...>();
-	auto __impl = [&]<size_t... _Indices>(index_sequence<_Indices...>)
-	{
-	  using _SubExtents = extents<_IndexType,
-	      (__mdspan::__static_slice_extent<_IndexType,
-		 _Extents...[__inv_map[_Indices]],
-		 _Slices...[__inv_map[_Indices]]>())...>;
-	  if constexpr (_SubExtents::rank_dynamic() == 0)
-	    return _SubExtents{};
-	  else
-	    {
-	      using _StaticSubExtents = __mdspan::_StaticExtents<
-		__mdspan::__static_extents<_SubExtents>()>;
-	      auto __create = [&]<size_t... _Is>(index_sequence<_Is...>)
-	      {
-		constexpr auto __slice_idx = [__inv_map](size_t __i) consteval
-		{
-		  return __inv_map[_StaticSubExtents::_S_dynamic_index_inv(__i)];
-		};
-
-		return _SubExtents{
-		  (__mdspan::__dynamic_slice_extent<__slice_idx(_Is)>(
-		     __exts, __slices...[__slice_idx(_Is)]))...};
-	      };
-	      constexpr auto __dyn_subrank = _SubExtents::rank_dynamic();
-	      return __create(make_index_sequence<__dyn_subrank>());
-	    }
-	};
-
-	return __impl(make_index_sequence<__inv_map.size()>());
+	__mdspan::__check_valid_slices(__mapping.extents(), __slices...);
+	return submdspan_mapping(__mapping, __slices...);
       }
   }
 
@@ -2792,6 +3218,25 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       };
       return __impl(__mdspan::__slice_cast<_IndexType>(__raw_slices)...);
     }
+
+  template<typename _ElementType, typename _Extents, typename _Layout,
+	   typename _Accessor, typename... _RawSlices>
+    requires (sizeof...(_RawSlices) == _Extents::rank()
+	&& __mdspan::__sliceable_mapping<typename _Layout::mapping<_Extents>,
+	                                 __mdspan::__full_extent_t<_RawSlices>...>)
+    constexpr auto
+    submdspan(
+	const mdspan<_ElementType, _Extents, _Layout, _Accessor>& __md,
+	_RawSlices... __raw_slices)
+    {
+      using _IndexType = typename _Extents::index_type;
+      auto [__mapping, __offset] = __mdspan::__submapping(
+	__md.mapping(), __mdspan::__slice_cast<_IndexType>(__raw_slices)...);
+      return std::mdspan(
+	       __md.accessor().offset(__md.data_handle(), __offset),
+	       std::move(__mapping),
+	       typename _Accessor::offset_policy(__md.accessor()));
+    }
 #endif // __glibcxx_submdspan
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/src/c++23/std.cc.in b/libstdc++-v3/src/c++23/std.cc.in
index 4962d4fc598..ef0da5d685a 100644
--- a/libstdc++-v3/src/c++23/std.cc.in
+++ b/libstdc++-v3/src/c++23/std.cc.in
@@ -1899,8 +1899,8 @@ export namespace std
   using std::submdspan_mapping_result;
   using std::submdspan_canonicalize_slices;
   using std::submdspan_extents;
+  using std::submdspan;
 #endif
-  // FIXME mdsubspan
 }
 #endif
 
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/generic.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/generic.cc
new file mode 100644
index 00000000000..682ff62254c
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/generic.cc
@@ -0,0 +1,71 @@
+// { dg-do compile { target c++26 } }
+#include <mdspan>
+
+namespace adl
+{
+  struct NoFriend
+  {
+    template<typename Extents>
+      class mapping
+      {
+      public:
+	using extents_type = Extents;
+	using index_type = typename extents_type::index_type;
+      };
+  };
+
+  struct NoFull
+  {
+    template<typename Extents>
+      class mapping
+      {
+      public:
+	using extents_type = Extents;
+	using index_type = typename extents_type::index_type;
+
+      private:
+	friend constexpr auto
+	submdspan_mapping(mapping, int)
+	{ return std::submdspan_mapping_result{mapping{}, 0}; }
+      };
+  };
+
+  struct WrongReturnValue
+  {
+    template<typename Extents>
+      class mapping
+      {
+      public:
+	using extents_type = Extents;
+	using index_type = typename extents_type::index_type;
+
+      private:
+	friend constexpr int
+	submdspan_mapping(mapping, std::full_extent_t)
+	{ return 42; }
+      };
+  };
+}
+
+template<typename MdSpan, typename... Slices>
+  concept submdspan_exists = requires (MdSpan md, Slices... slices)
+  {
+    std::submdspan(md, slices...);
+  };
+
+template<typename Layout, bool Expected>
+constexpr bool
+test_invalid_mapping()
+{
+  using Extents = std::extents<int, 3>;
+  using MdSpan = std::mdspan<double, Extents, Layout>;
+  static_assert(submdspan_exists<MdSpan, int> == Expected);
+  static_assert(submdspan_exists<MdSpan, std::full_extent_t> == Expected);
+  static_assert(!submdspan_exists<MdSpan>);
+  static_assert(!submdspan_exists<MdSpan, int, int>);
+  return true;
+}
+static_assert(test_invalid_mapping<std::layout_left, true>());
+static_assert(test_invalid_mapping<adl::NoFriend, false>());
+static_assert(test_invalid_mapping<adl::NoFull, false>());
+static_assert(test_invalid_mapping<adl::WrongReturnValue, false>());
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left.cc
new file mode 100644
index 00000000000..d6a85d0380b
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/left.cc
@@ -0,0 +1,9 @@
+// { dg-do run { target c++26 } }
+#include "testcases.h"
+
+int
+main()
+{
+  test_all<std::layout_left>();
+  return 0;
+}
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/testcases.h b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/testcases.h
new file mode 100644
index 00000000000..d7b751d700c
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/selections/testcases.h
@@ -0,0 +1,360 @@
+#include <mdspan>
+
+#include <vector>
+#include <numeric>
+#include "../../layout_traits.h"
+#include <testsuite_hooks.h>
+
+constexpr size_t dyn = std::dynamic_extent;
+constexpr auto all = std::full_extent;
+
+template<typename T>
+  constexpr bool is_strided_slice = false;
+
+template<typename O, typename E, typename S>
+  constexpr bool is_strided_slice<std::strided_slice<O, E, S>> = true;
+
+template<typename MDSpan>
+  constexpr void
+  fill(const MDSpan& md)
+  {
+    using IndexType = typename MDSpan::index_type;
+    auto exts = md.extents();
+    if constexpr (exts.rank() == 3)
+      for(IndexType i = 0; i < exts.extent(0); ++i)
+	for(IndexType j = 0; j < exts.extent(1); ++j)
+	  for(IndexType k = 0; k < exts.extent(2); ++k)
+	    md[i, j, k] = 100 * i + 10 * j + k;
+  }
+
+template<typename Int, size_t Rank>
+  class multi_index_generator
+  {
+    struct sentinel
+    { };
+
+    class iterator
+    {
+    public:
+      constexpr
+      iterator(const std::array<Int, Rank>& shape)
+	: M_shape(shape)
+      { }
+
+      constexpr iterator&
+      operator++()
+      {
+	if constexpr (Rank > 0)
+	  {
+	    ++M_indices[Rank-1];
+	    for(size_t i = Rank; i > 1; --i)
+	      if (M_indices[i-1] == M_shape[i-1])
+		{
+		  M_indices[i-1] = 0;
+		  ++M_indices[i-2];
+		}
+	  }
+	return *this;
+      }
+
+      constexpr auto
+      operator*()
+      { return M_indices; }
+
+    private:
+      friend constexpr bool
+      operator==(const iterator& it, sentinel)
+      {
+	if constexpr (Rank > 0)
+	  return it.M_indices[0] == it.M_shape[0];
+	else
+	  return true;
+      }
+
+      std::array<Int, Rank> M_indices{};
+      std::array<Int, Rank> M_shape;
+    };
+
+  public:
+    constexpr
+    multi_index_generator(std::array<Int, Rank> shape)
+      : M_shape(shape)
+    { }
+
+    constexpr iterator
+    begin() const
+    { return iterator(M_shape); }
+
+    constexpr sentinel
+    end() const
+    { return sentinel{}; }
+
+  private:
+    std::array<Int, Rank> M_shape;
+  };
+
+constexpr bool
+test_multi_index()
+{
+  auto shape = std::array{3, 5, 7, 1};
+  auto gen = multi_index_generator(shape);
+  auto it = gen.begin();
+  auto end = gen.end();
+  for (int i = 0; i < shape[0]; ++i)
+    for (int j = 0; j < shape[1]; ++j)
+      for (int k = 0; k < shape[2]; ++k)
+	for (int l = 0; l < shape[3]; ++l)
+	  {
+	    VERIFY(it != end);
+	    VERIFY(*it == std::array{i, j, k, l});
+	    ++it;
+	  }
+  return true;
+}
+static_assert(test_multi_index());
+
+struct
+collapse
+{ };
+
+template<typename... Slices>
+  consteval auto
+  inv_collapsed_index_map()
+  {
+    constexpr size_t rank = sizeof...(Slices);
+    auto is_collapsing = std::array{std::same_as<Slices, collapse>...};
+    constexpr auto collapsed_rank = ((!std::same_as<Slices, collapse>) + ... + 0);
+
+    std::array<size_t, collapsed_rank> ret;
+    if constexpr (collapsed_rank > 0)
+      for(size_t k = 0, i = 0; i < rank; ++i)
+	if (!is_collapsing[i])
+	  ret[k++] = i;
+    return ret;
+  }
+
+static_assert(inv_collapsed_index_map<collapse, collapse, collapse>()
+	      == std::array<size_t, 0>{});
+
+static_assert(inv_collapsed_index_map<collapse, decltype(all), collapse>()
+	      == std::array<size_t, 1>{1});
+
+template<typename IndexType, typename Slice>
+  constexpr std::vector<IndexType>
+  make_selection(IndexType extent, const Slice& slice)
+  {
+    if constexpr (std::convertible_to<Slice, IndexType>)
+      return {static_cast<IndexType>(slice)};
+    else if constexpr (std::same_as<Slice, std::full_extent_t>)
+      {
+	auto ret = std::vector<IndexType>(static_cast<size_t>(extent));
+	std::ranges::iota(ret, 0);
+	return ret;
+      }
+    else if constexpr (is_strided_slice<Slice>)
+      {
+	auto ret = std::vector<IndexType>{};
+	size_t n = static_cast<size_t>(slice.extent);
+	for(size_t i = 0; i < n; i += slice.stride)
+	  ret.push_back(slice.offset + i);
+	return ret;
+      }
+    else
+      {
+	auto [begin, end] = slice;
+	auto ret = std::vector<IndexType>(static_cast<size_t>(end - begin));
+	std::ranges::iota(ret, begin);
+	return ret;
+      }
+  }
+
+template<typename Layout, size_t... I, typename... Slices>
+  constexpr bool
+  check_selection(std::index_sequence<I...>, auto md, Slices... slices)
+  {
+    auto exts = md.extents();
+    auto outer_shape = std::array{exts.extent(0), exts.extent(1), exts.extent(2)};
+
+    constexpr auto full_index = inv_collapsed_index_map<Slices...>();
+    auto make_slice = [](size_t i, auto slice)
+    {
+      if constexpr (std::same_as<decltype(slice), collapse>)
+	return i;
+      else
+	return slice;
+    };
+
+    auto loop_body = [&]<size_t... J>(std::index_sequence<J...>, auto ijk,
+				      auto... slices)
+    {
+      auto submd = submdspan(md, slices...[I]...);
+      auto selection = std::tuple{make_selection(exts.extent(I), slices...[I])...};
+      auto inner_shape = std::array<size_t, full_index.size()>{
+	std::get<full_index[J]>(selection).size()...
+      };
+
+      for (auto ij : multi_index_generator(inner_shape))
+      {
+	((ijk[full_index[J]] = get<full_index[J]>(selection)[ij[J]]),...);
+	VERIFY(submd[ij] == md[ijk]);
+      }
+    };
+
+    for (auto ijk : multi_index_generator(outer_shape))
+      loop_body(std::make_index_sequence<full_index.size()>(), ijk,
+		make_slice(ijk[I], slices...[I])...);
+    return true;
+  }
+
+template<typename Layout, typename...MD, typename... Slices>
+  constexpr bool
+  check_selection(std::mdspan<MD...> md, Slices... slices)
+  {
+    auto indices = std::make_index_sequence<sizeof...(slices)>();
+    return check_selection<Layout>(indices, md, slices...);
+  }
+
+template<typename Layout, typename IndexType, size_t... Extents,
+	 typename... Slices>
+  constexpr bool
+  check_selection(std::extents<IndexType, Extents...>exts, Slices... slices)
+  {
+    auto run = [&](auto m)
+    {
+      auto storage = std::vector<double>(m.required_span_size());
+      auto md = std::mdspan(storage.data(), m);
+      fill(md);
+      return check_selection<Layout>(md, slices...);
+    };
+
+    if constexpr (std::same_as<Layout, std::layout_stride>)
+      {
+	auto m = typename Layout::mapping(exts, std::array{15, 2, 50});
+	return run(m);
+      }
+    else
+      {
+	auto m = typename Layout::mapping(exts);
+	return run(m);
+      }
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_scalar_selection(auto exts)
+  {
+    check_selection<Layout>(exts, collapse{}, collapse{}, collapse{});
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_full_lines(auto exts)
+  {
+    check_selection<Layout>(exts, all, collapse{}, collapse{});
+    check_selection<Layout>(exts, collapse{}, all, collapse{});
+    check_selection<Layout>(exts, collapse{}, collapse{}, all);
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_full_blocks(auto exts)
+  {
+    check_selection<Layout>(exts, all, all, collapse{});
+    check_selection<Layout>(exts, all, collapse{}, all);
+    check_selection<Layout>(exts, collapse{}, all, all);
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_cubes(auto exts)
+  {
+    auto s0 = std::pair{0, 2};
+    auto s1 = std::pair{1, 4};
+    auto s2 = std::pair{3, 7};
+
+    check_selection<Layout>(exts, all, all, all);
+    check_selection<Layout>(exts, all, all, s2);
+    check_selection<Layout>(exts, s0, all, all);
+    check_selection<Layout>(exts, s0, all, s2);
+    check_selection<Layout>(exts, s0, s1, s2);
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_line_selection(auto exts)
+  {
+    auto check = [&](auto s)
+    {
+      check_selection<Layout>(exts, collapse{}, s, collapse{});
+    };
+
+    check(std::strided_slice(0, 2, 2));
+    check(std::strided_slice(0, 3, 2));
+    check(std::strided_slice(1, 3, 2));
+    check(std::strided_slice(1, std::cw<3>, std::cw<2>));
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_box_selection(auto exts)
+  {
+    auto s0 = std::strided_slice(0, 3, 2);
+    auto s1 = std::strided_slice(1, 4, 2);
+    auto s2 = std::strided_slice(0, 7, 3);
+
+    check_selection<Layout>(exts, s0, s1, s2);
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_all_cheap()
+  {
+    constexpr auto dyn_exts = std::extents(3, 5, 7);
+    constexpr auto sta_exts = std::extents<int, 3, 5, 7>{};
+
+    test_scalar_selection<Layout>(dyn_exts);
+    test_scalar_selection<Layout>(sta_exts);
+    static_assert(test_scalar_selection<Layout>(dyn_exts));
+    static_assert(test_scalar_selection<Layout>(sta_exts));
+
+    test_full_lines<Layout>(dyn_exts);
+    test_full_lines<Layout>(sta_exts);
+    static_assert(test_full_lines<Layout>(dyn_exts));
+    static_assert(test_full_lines<Layout>(sta_exts));
+
+    test_strided_box_selection<Layout>(dyn_exts);
+    test_strided_box_selection<Layout>(sta_exts);
+    static_assert(test_strided_box_selection<Layout>(dyn_exts));
+    static_assert(test_strided_box_selection<Layout>(sta_exts));
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_all_expensive()
+  {
+    auto run = [](auto exts)
+    {
+      test_full_blocks<Layout>(exts);
+      test_cubes<Layout>(exts);
+    };
+
+    run(std::extents(3, 5, 7));
+    run(std::extents<int, 3, 5, 7>{});
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_all()
+  {
+    test_all_cheap<Layout>();
+    test_all_expensive<Layout>();
+    return true;
+  }
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
new file mode 100644
index 00000000000..a37d3cd588f
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_mapping.cc
@@ -0,0 +1,136 @@
+// { dg-do run { target c++26 } }
+#include <mdspan>
+
+#include <iostream> // TODO remove
+#include "../layout_traits.h"
+#include <testsuite_hooks.h>
+
+constexpr size_t dyn = std::dynamic_extent;
+
+template<typename Mapping, typename... Slices>
+  constexpr auto
+  call_submdspan_mapping(const Mapping& m, std::tuple<Slices...> slices)
+  {
+    auto impl = [&]<size_t... I>(std::index_sequence<I...>)
+    { return submdspan_mapping(m, get<I>(slices)...); };
+    return impl(std::make_index_sequence<sizeof...(Slices)>());
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_layout_unpadded_return_types()
+  {
+    constexpr auto padding_side = DeducePaddingSide::from_typename<Layout>();
+    using Traits = LayoutTraits<padding_side>;
+
+    {
+      auto m0 = typename Layout::mapping(std::extents());
+      auto result = submdspan_mapping(m0);
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, Layout>);
+    }
+
+    auto exts = Traits::make_extents(std::dims<5, int>(3, 5, 7, 11, 13));
+    auto m = typename Layout::mapping(exts);
+    auto all = std::full_extent;
+    auto s251 = std::strided_slice{2, 5, std::cw<1>};
+
+    {
+      auto slices = std::tuple{0, 0, 0, 0, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, Layout>);
+    }
+
+    {
+      auto slices = std::tuple{all, all, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, Layout>);
+    }
+
+    {
+      auto s0 = std::strided_slice{1, 1, std::cw<1>};
+      auto slices = std::tuple{s0, all, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(is_same_padded<padding_side, layout_type>);
+    }
+
+    {
+      auto s0 = std::strided_slice{1, 2, std::cw<1>};
+      auto slices = std::tuple{s0, all, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(is_same_padded<padding_side, layout_type>);
+    }
+
+    {
+      auto s0 = std::strided_slice{1, 2, std::cw<1>};
+      auto slices = std::tuple{s0, 0, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = typename decltype(result.mapping)::layout_type;
+      static_assert(is_same_padded<padding_side, layout_type>);
+    }
+
+    {
+      auto s0 = std::strided_slice{1, 2, 1};
+      auto slices = std::tuple{s0, all, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, std::layout_stride>);
+    }
+
+    {
+      auto slices = std::tuple{1, all, all, s251, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, std::layout_stride>);
+    }
+
+    {
+      auto s3 = std::strided_slice{2, std::cw<7>, std::cw<2>};
+      auto slices = std::tuple{all, all, all, s3, 0};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      using layout_type = decltype(result.mapping)::layout_type;
+      static_assert(std::same_as<layout_type, std::layout_stride>);
+    }
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_layout_unpadded_padding_value()
+  {
+    using Traits = LayoutTraits<DeducePaddingSide::from_typename<Layout>()>;
+    auto s0 = std::strided_slice{size_t(1), size_t(2), std::cw<size_t(1)>};
+    auto s3 = std::strided_slice{size_t(2), size_t(5), std::cw<size_t(1)>};
+    auto all = std::full_extent;
+
+    auto check = [&](auto exts, size_t expected)
+    {
+      auto m = typename Layout::mapping(Traits::make_extents(exts));
+      auto slices = std::tuple{s0, size_t(0), all, s3, size_t(0)};
+      auto result = call_submdspan_mapping(m, Traits::make_tuple(slices));
+      auto padding_value = decltype(result.mapping)::padding_value;
+      VERIFY(padding_value == expected);
+    };
+
+    check(std::extents(std::cw<3>, std::cw<5>, std::cw<7>, 11, 13), 3*5);
+    check(std::extents(std::cw<3>, std::cw<5>, 7, 11, 13), 3*5);
+    check(std::extents(std::cw<3>, 5, 7, 11, 13), dyn);
+    check(std::extents(3, 5, 7, 11, 13), dyn);
+    return true;
+  }
+
+int
+main()
+{
+  test_layout_unpadded_return_types<std::layout_left>();
+  static_assert(test_layout_unpadded_return_types<std::layout_left>());
+
+  test_layout_unpadded_padding_value<std::layout_left>();
+  static_assert(test_layout_unpadded_padding_value<std::layout_left>());
+  return 0;
+}
+
diff --git a/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
new file mode 100644
index 00000000000..1fc10a832eb
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/mdspan/submdspan/submdspan_neg.cc
@@ -0,0 +1,104 @@
+// { dg-do compile { target c++26 } }
+#include <mdspan>
+
+#include <vector>
+
+template<typename Layout, typename... Slices>
+  constexpr bool
+  check_slice_range(Slices... slices)
+  {
+    auto m = typename Layout::mapping<std::extents<int, 3, 5, 7>>{};
+    auto storage = std::vector<double>(m.required_span_size());
+    auto md = std::mdspan(storage.data(), m);
+
+    auto submd = submdspan(md, slices...);           // { dg-error "expansion of" }
+    (void) submd;
+    return true;
+  }
+
+template<typename Layout>
+  constexpr bool
+  test_int_under()
+  {
+    check_slice_range<Layout>(1, -1, 2);             // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_int_under<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_int_over()
+  {
+    check_slice_range<Layout>(1, 5, 2);              // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_int_over<std::layout_left>());    // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_tuple_under()
+  {
+    check_slice_range<Layout>(1, std::tuple{-1, 2}, 2);  // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_tuple_under<std::layout_left>());     // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_tuple_reversed()
+  {
+    check_slice_range<Layout>(1, std::tuple{3, 2}, 2);   // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_tuple_reversed<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_tuple_over()
+  {
+    check_slice_range<Layout>(1, std::tuple{0, 6}, 2); // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_tuple_over<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_slice_zero()
+  {
+    check_slice_range<Layout>(1, std::strided_slice{1, 1, 0}, 2);  // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_strided_slice_zero<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_slice_offset_under()
+  {
+    check_slice_range<Layout>(1, std::strided_slice{-1, 1, 1}, 2);   // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_strided_slice_offset_under<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_slice_offset_over()
+  {
+    check_slice_range<Layout>(1, std::strided_slice{6, 0, 1}, 2);    // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_strided_slice_offset_over<std::layout_left>());   // { dg-error "expansion of" }
+
+template<typename Layout>
+  constexpr bool
+  test_strided_slice_extent_over()
+  {
+    check_slice_range<Layout>(1, std::strided_slice{1, 5, 1}, 2);    // { dg-error "expansion of" }
+    return true;
+  }
+static_assert(test_strided_slice_extent_over<std::layout_left>());   // { dg-error "expansion of" }
+
+// { dg-prune-output "static assertion failed" }
+// { dg-prune-output "__glibcxx_assert_fail" }
+// { dg-prune-output "non-constant condition" }
+// { dg-prune-output "no matching function" }
+// { dg-prune-output "does not satisfy placeholder constraints" }