[libc++] mdspan - implement layout_stride

[crtrott]

Summary

This implements layout_stride for C++23 and with that completes the implementation of the C++23 mdspan header.
The feature test macro is added, and the status pages updated.

Co-authored-by: Damien L-G <[e-mail]>

[crtrott]

Fix up comments for some formatting check.

[crtrott]

More formatting fixes in the tests.

[crtrott]

Fix ADL use warning.

[crtrott]

More fixes for things not triggered in my various MacOS builds.

[crtrott]

Fixup module map

[Mordante]

libcxx/include/__mdspan/layout_stride.h

19

The file modules/std/mdspan.cppm should be updated; uncomment using std::layout_stride;

[crtrott]

Update mdspan std::module file

[crtrott]

Fix older clang missing a typename in a test

[crtrott]

This passed testing except the expected AIX failure since the runners are down.

[philnik]

libcxx/docs/Status/Cxx23Papers.csv

94–96

These should all be completed with this patch too.

libcxx/include/__mdspan/layout_stride.h

149

Throughout. @var-const Do you have any thoughts here?

164

228

Maybe?

268

291

Test!

312

Where does this come from?

314

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.conversion.pass.cpp

80

Maybe try to trigger the edge case here? Just ignore the comment if it doesn't work.

84

Possible future work: Add tests to make sure that mdspan and friends work with _BitInt.

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_array.pass.cpp

53

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_span.pass.cpp

54

83

This should be a span!

libcxx/test/std/containers/views/mdspan/CustomTestLayouts.h

278

libcxx/test/std/containers/views/mdspan/layout_stride/comparison.pass.cpp

50–56

template <class E1, class E2>
concept layout_mapping_comparble = requires(E1 e1, E2 e2) { std::layout_stride::mapping<E1>(e1, s1) == std::layout_stride::mapping<E2>(e2, s2); }

should also work, right?

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.default.pass.cpp

38

Please add a test to make sure extents() is const qualified. Also for required_span_size() and strides().

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.extents_array.pass.cpp

2

Note to self: Continue here.

[crtrott]

Address review feedback, and implement missing ctors from layout_stride in layout_left/right

[crtrott]

Fix a formatting thing in tests.

[crtrott]

Fix more warning stuff

[crtrott]

libcxx/include/__mdspan/layout_stride.h

228

Results in warning: include/c++/v1/__mdspan/layout_stride.h:322:49: error: left operand of comma operator has no effect [-Werror,-Wunused-value]

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.conversion.pass.cpp

84

Will do this in a future revision for all the relevant pieces starting with extents.

[crtrott]

Add missing test on const mapping for required_span_size

[crtrott]

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.default.pass.cpp

38

Done in properties.pass.cpp

[crtrott]

Simply disable deduction guide warnings, adding them explicitly ended up either with unused template warning in files where they are not needed, or with a warning from GCC if one adds [[maybe_unused]], will try separately to make this warning go away.

[ldionne]

libcxx/include/__mdspan/layout_stride.h

164

I would suggest pulling this into a __bubble_sort helper function and moving the comment there. That way, if we want to change the approach later on (e.g. use std::sort for real), it'll be easier. The code will also get a lot simpler here.

197

231

I think it would make sense to create an __offset function of some kind for https://eel.is/c++draft/mdspan.layout#stride.expo-2. It should handle the case where one of the extents is 0. Also probably needs a test.

Good catch!

285

314

This should be a LWG issue? It should be pretty simple.

326

This is __offset again. Is it wrong here too? If wrong, this is missing a test.

libcxx/test/libcxx/containers/views/mdspan/layout_left/assert.ctor.layout_stride.pass.cpp

13 ↗

(On Diff #551188)

Can you link to the LLVM issue you created about this? We should have a way to do this with _LIBCPP_CTAD_SUPPORTED_FOR_TYPE.

libcxx/test/std/containers/views/mdspan/layout_right/ctor.layout_stride.pass.cpp

33–36 ↗

(On Diff #551188)

That way you can't mess up the test by calling it with different arguments.

[crtrott]

libcxx/test/libcxx/containers/views/mdspan/layout_left/assert.ctor.layout_stride.pass.cpp

13 ↗

(On Diff #551188)

https://github.com/llvm/llvm-project/issues/64719

[crtrott]

Address more review comments: fix some assertions when extents are 0

[crtrott]

Fix a last few review comments.

[ldionne]

I have a few more comments but this pretty much LGTM assuming CI passes.

libcxx/test/libcxx/containers/views/mdspan/layout_left/assert.ctor.layout_stride.pass.cpp

13 ↗

(On Diff #551188)

Sorry, I meant add a comment in the test so that we can remove the -Wno-ctad-... at some point.

libcxx/test/libcxx/containers/views/mdspan/layout_right/assert.ctor.layout_stride.pass.cpp

62 ↗

(On Diff #557718)

Is this what you mean or is the test wrong?

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.index_operator.pass.cpp

12

Shouldn't these assertions be enabled in the hardened mode too?

libcxx/test/std/containers/views/mdspan/layout_left/ctor.layout_stride.pass.cpp

55 ↗

(On Diff #557718)

I don't understand why you are performing an assignment here. Aren't we trying to test construction?

Also, the following should be sufficient to check for implicit constructibility:

To dest = src;

this should allow getting rid of test_implicit_conversion. Yes, I left a comment last time to change to the lambda but I guess I had not realized you could simplify this even more altogether.

[crtrott]

Address latest feedback

[crtrott]

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.index_operator.pass.cpp

12

I think we agreed to do it only in debug mode, because for the vast majority of use cases the assertion will be caught inside mdspan already, since most people would use layouts only through mdspan (and later mdarray).

libcxx/docs/FeatureTestMacroTable.rst

.. table:: Current Status
     ``__cpp_lib_formatters``                            *unimplemented*
     --------------------------------------------------- -----------------
     ``__cpp_lib_forward_like``                          ``202207L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_invoke_r``                              ``202106L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_is_scoped_enum``                        ``202011L``
     --------------------------------------------------- -----------------
-    ``__cpp_lib_mdspan``                                *unimplemented*
+    ``__cpp_lib_mdspan``                                ``202207L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_move_only_function``                    *unimplemented*
     --------------------------------------------------- -----------------
     ``__cpp_lib_optional``                              ``202110L``
     --------------------------------------------------- -----------------
     ``__cpp_lib_out_ptr``                               *unimplemented*
     --------------------------------------------------- -----------------
     ``__cpp_lib_print``                                 *unimplemented*

libcxx/docs/Status/Cxx23.rst

 
 .. csv-table::
    :file: Cxx23Papers.csv
    :header-rows: 1
    :widths: auto
 
 .. note::
 
-   .. [#note-P0009R18] P0009R18: ``extents``, ``dextents``, ``layout_left``, ``layout_right``, and ``default_accessor`` are implemented.
    .. [#note-P0533R9] P0533R9: ``isfinite``, ``isinf``, ``isnan`` and ``isnormal`` are implemented.
    .. [#note-P1413R3] P1413R3: ``std::aligned_storage_t`` and ``std::aligned_union_t`` are marked deprecated, but
       clang doesn't issue a diagnostic for deprecated using template declarations.
    .. [#note-P2520R0] P2520R0: Libc++ implemented this paper as a DR in C++20 as well.
    .. [#note-P2711R1] P2711R1: ``join_with_view`` hasn't been done yet since this type isn't implemented yet.
    .. [#note-P2693R1] P2693R1: The formatter for ``std::thread::id`` is implemented.
       The formatter for ``stacktrace`` is not implemented, since ``stacktrace`` is
       not implemented yet.

libcxx/docs/Status/Cxx23Papers.csv

 "`P2255R2 <https://wg21.link/P2255R2>`__","LWG","A type trait to detect reference binding to temporary","February 2022","",""
 "`P2273R3 <https://wg21.link/P2273R3>`__","LWG","Making ``std::unique_ptr`` constexpr","February 2022","|Complete|","16.0"
 "`P2387R3 <https://wg21.link/P2387R3>`__","LWG","Pipe support for user-defined range adaptors","February 2022","","","|ranges|"
 "`P2440R1 <https://wg21.link/P2440R1>`__","LWG","``ranges::iota``, ``ranges::shift_left`` and ``ranges::shift_right``","February 2022","","","|ranges|"
 "`P2441R2 <https://wg21.link/P2441R2>`__","LWG","``views::join_with``","February 2022","","","|ranges|"
 "`P2442R1 <https://wg21.link/P2442R1>`__","LWG","Windowing range adaptors: ``views::chunk`` and ``views::slide``","February 2022","","","|ranges|"
 "`P2443R1 <https://wg21.link/P2443R1>`__","LWG","``views::chunk_by``","February 2022","","","|ranges|"
 "","","","","","",""
-"`P0009R18 <https://wg21.link/P0009R18>`__","LWG","mdspan: A Non-Owning Multidimensional Array Reference","July 2022","|In progress| [#note-P0009R18]_|",""
+"`P0009R18 <https://wg21.link/P0009R18>`__","LWG","mdspan: A Non-Owning Multidimensional Array Reference","July 2022","|Complete|",""
 "`P0429R9 <https://wg21.link/P0429R9>`__","LWG","A Standard ``flat_map``","July 2022","",""
 "`P1169R4 <https://wg21.link/P1169R4>`__","LWG","``static operator()``","July 2022","|Complete|","16.0"
 "`P1222R4 <https://wg21.link/P1222R4>`__","LWG","A Standard ``flat_set``","July 2022","",""
 "`P1223R5 <https://wg21.link/P1223R5>`__","LWG","``ranges::find_last()``, ``ranges::find_last_if()``, and ``ranges::find_last_if_not()``","July 2022","","","|ranges|"
 "`P1467R9 <https://wg21.link/P1467R9>`__","LWG","Extended ``floating-point`` types and standard names","July 2022","",""
 "`P1642R11 <https://wg21.link/P1642R11>`__","LWG","Freestanding ``[utilities]``, ``[ranges]``, and ``[iterators]``","July 2022","",""
 "`P1899R3 <https://wg21.link/P1899R3>`__","LWG","``stride_view``","July 2022","","","|ranges|"
 "`P2093R14 <https://wg21.link/P2093R14>`__","LWG","Formatted output","July 2022","","|In Progress|"

 "`P2517R1 <https://wg21.link/P2517R1>`__","LWG","Add a conditional ``noexcept`` specification to ``std::apply``","July 2022","",""
 "`P2520R0 <https://wg21.link/P2520R0>`__","LWG","``move_iterator`` should be a random access iterator","July 2022","|Complete| [#note-P2520R0]_","17.0","|ranges|"
 "`P2540R1 <https://wg21.link/P2540R1>`__","LWG","Empty Product for certain Views","July 2022","","","|ranges|"
 "`P2549R1 <https://wg21.link/P2549R1>`__","LWG","``std::unexpected`` should have ``error()`` as member accessor","July 2022","|Complete|","16.0"
 "`P2553R1 <https://wg21.link/P2553R1>`__","LWG","Make ``mdspan`` ``size_type`` controllable","July 2022","",""
 "`P2554R0 <https://wg21.link/P2554R0>`__","LWG","C-Array Interoperability of MDSpan","July 2022","",""
 "`P2585R0 <https://wg21.link/P2585R0>`__","LWG","Improving default container formatting","July 2022","|Complete|","17.0"
 "`P2590R2 <https://wg21.link/P2590R2>`__","LWG","Explicit lifetime management","July 2022","",""
 "`P2599R2 <https://wg21.link/P2599R2>`__","LWG","``mdspan::size_type`` should be ``index_type``","July 2022","",""
 "`P2604R0 <https://wg21.link/P2604R0>`__","LWG","mdspan: rename pointer and contiguous","July 2022","",""
 "`P2613R1 <https://wg21.link/P2613R1>`__","LWG","Add the missing ``empty`` to ``mdspan``","July 2022","",""
 "","","","","","",""
 "`P1202R5 <https://wg21.link/P1202R5>`__","LWG", "Asymmetric Fences", "November 2022","","","|concurrency TS|"
 "`P1264R2 <https://wg21.link/P1264R2>`__","LWG", "Revising the wording of ``stream`` input operations", "November 2022","|Complete|","9.0",""
 "`P1478R8 <https://wg21.link/P1478R8>`__","LWG", "``Byte-wise`` ``atomic`` ``memcpy``", "November 2022","","","|concurrency TS|"
 "`P2167R3 <https://wg21.link/P2167R3>`__","LWG", "Improved Proposed Wording for LWG 2114", "November 2022","","",""
 "`P2396R1 <https://wg21.link/P2396R1>`__","LWG", "Concurrency TS 2 fixes ", "November 2022","","","|concurrency TS|"
 "`P2505R5 <https://wg21.link/P2505R5>`__","LWG", "Monadic Functions for ``std::expected``", "November 2022","|Complete|","17.0",""
 "`P2539R4 <https://wg21.link/P2539R4>`__","LWG", "Should the output of ``std::print`` to a terminal be synchronized with the underlying stream?", "November 2022","|In Progress|","","|format|"

libcxx/include/CMakeLists.txt

set(files
   __math/rounding_functions.h
   __math/traits.h
   __math/trigonometric_functions.h
   __mbstate_t.h
   __mdspan/default_accessor.h
   __mdspan/extents.h
   __mdspan/layout_left.h
   __mdspan/layout_right.h
+  __mdspan/layout_stride.h
   __mdspan/mdspan.h
   __memory/addressof.h
   __memory/align.h
   __memory/aligned_alloc.h
   __memory/allocate_at_least.h
   __memory/allocation_guard.h
   __memory/allocator.h
   __memory/allocator_arg_t.h

libcxx/include/__fwd/mdspan.h

 };
 
 // Layout policy with a mapping which corresponds to C-style array layouts
 struct layout_right {
   template <class _Extents>
   class mapping;
 };
 
-/*
-// Will be implemented with follow on revision
 // Layout policy with a unique mapping where strides are arbitrary
 struct layout_stride {
-  template<class Extents>
-    class mapping;
+  template <class Extents>
 };
-*/
 
 #endif // _LIBCPP_STD_VER >= 23
 
 _LIBCPP_END_NAMESPACE_STD
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___MDSPAN_LAYOUTS_H

libcxx/include/__mdspan/layout_stride.h

+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//                        Kokkos v. 4.0
+//       Copyright (2022) National Technology & Engineering
+//               Solutions of Sandia, LLC (NTESS).
+//
+// Under the terms of Contract DE-NA0003525 with NTESS,
+// the U.S. Government retains certain rights in this software.
+//
+//===---------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___MDSPAN_LAYOUT_STRIDE_H
+#define _LIBCPP___MDSPAN_LAYOUT_STRIDE_H
+
+#include <__assert>
+#include <__config>
+#include <__fwd/mdspan.h>
+#include <__mdspan/extents.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_convertible.h>
+#include <__type_traits/is_nothrow_constructible.h>
+#include <__utility/as_const.h>
+#include <__utility/integer_sequence.h>
+#include <array>
+#include <cinttypes>
+#include <cstddef>
+#include <limits>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#if _LIBCPP_STD_VER >= 23
+
+namespace __mdspan_detail {
+template <class _Layout, class _Mapping>
+constexpr bool __is_mapping_of =
+    is_same_v<typename _Layout::template mapping<typename _Mapping::extents_type>, _Mapping>;
+
+template <class _Mapping>
+concept __layout_mapping_alike = requires {
+  requires __is_mapping_of<typename _Mapping::layout_type, _Mapping>;
+  requires __is_extents_v<typename _Mapping::extents_type>;
+  { _Mapping::is_always_strided() } -> same_as<bool>;
+  { _Mapping::is_always_exhaustive() } -> same_as<bool>;
+  { _Mapping::is_always_unique() } -> same_as<bool>;
+  bool_constant<_Mapping::is_always_strided()>::value;
+  bool_constant<_Mapping::is_always_exhaustive()>::value;
+  bool_constant<_Mapping::is_always_unique()>::value;
+};
+} // namespace __mdspan_detail
+
+template <class _Extents>
+class layout_stride::mapping {
+public:
+  static_assert(__mdspan_detail::__is_extents<_Extents>::value,
+                "layout_stride::mapping template argument must be a specialization of extents.");
+
+  using extents_type = _Extents;
+  using index_type   = typename extents_type::index_type;
+  using size_type    = typename extents_type::size_type;
+  using rank_type    = typename extents_type::rank_type;
+  using layout_type  = layout_stride;
+
+private:
+  static constexpr rank_type __rank_ = extents_type::rank();
+
+  // Used for default construction check and mandates
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __required_span_size_is_representable(const extents_type& __ext) {
+    if constexpr (__rank_ == 0)
+      return true;
+
+    index_type __prod = __ext.extent(0);
+    for (rank_type __r = 1; __r < __rank_; __r++) {
+      bool __overflowed = __builtin_mul_overflow(__prod, __ext.extent(__r), &__prod);
+      if (__overflowed)
+        return false;
+    }
+    return true;
+  }
+
+  template <class _OtherIndexType>
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool
+  __required_span_size_is_representable(const extents_type& __ext, span<_OtherIndexType, __rank_> __strides) {
+    if constexpr (__rank_ == 0)
+      return true;
+
+    index_type __size = 1;
+    for (rank_type __r = 0; __r < __rank_; __r++) {
+      // We can only check correct conversion of _OtherIndexType if it is an integral
+      if constexpr (is_integral_v<_OtherIndexType>) {
+        using _CommonType = common_type_t<index_type, _OtherIndexType>;
+        if (static_cast<_CommonType>(__strides[__r]) > static_cast<_CommonType>(std::numeric_limits<index_type>::max()))
+          return false;
+      }
+      if (__ext.extent(__r) == static_cast<index_type>(0))
+        return true;
+      index_type __prod     = (__ext.extent(__r) - 1);
+      bool __overflowed_mul = __builtin_mul_overflow(__prod, static_cast<index_type>(__strides[__r]), &__prod);
+      if (__overflowed_mul)
+        return false;
+      bool __overflowed_add = __builtin_add_overflow(__size, __prod, &__size);
+      if (__overflowed_add)
+        return false;
+    }
+    return true;
+  }
+
+  static_assert((extents_type::rank_dynamic() > 0) || __required_span_size_is_representable(extents_type()),
+                "layout_stride::mapping product of static extents must be representable as index_type.");
+
+public:
+  // [mdspan.layout.stride.cons], constructors
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping() noexcept : __extents_(extents_type()) {
+    // Note the nominal precondition is covered by above static assert since
+    // if rank_dynamic is != 0 required_span_size is zero for default construction
+    if constexpr (__rank_ > 0) {
+      index_type __stride = 1;
+      for (rank_type __r = __rank_ - 1; __r > static_cast<rank_type>(0); __r--) {
+        __strides_[__r] = __stride;
+        __stride *= __extents_.extent(__r);
+      }
+      __strides_[0] = __stride;
+    }
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping(const mapping&) noexcept = default;
+
+  template <class _OtherIndexType>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping(const extents_type& __ext, span<_OtherIndexType, __rank_> __strides) noexcept
+      : __extents_(__ext), __strides_([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+          return __mdspan_detail::__possibly_empty_array<index_type, __rank_>{
+              static_cast<index_type>(std::as_const(__strides[_Pos]))...};
+        }(make_index_sequence<__rank_>())) {
+    _LIBCPP_ASSERT(([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+                     // For integrals we can do a pre-conversion check, for other types not
+                     if constexpr (is_integral_v<_OtherIndexType>) {
+                       return ((__strides[_Pos] > static_cast<_OtherIndexType>(0)) && ... && true);
+                     } else {
+                       return ((static_cast<index_type>(__strides[_Pos]) > static_cast<index_type>(0)) && ... && true);
+                     }
+                   }(make_index_sequence<__rank_>())),
+                   "layout_stride::mapping ctor: all strides must be greater than 0");
+    _LIBCPP_ASSERT(__required_span_size_is_representable(__ext, __strides),
+                   "layout_stride::mapping ctor: required span size is not representable as index_type.");
+    if constexpr (__rank_ > 1) {
+      _LIBCPP_ASSERT(
+          ([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+            // basically sort the dimensions based on strides and extents, sorting is represented in permute array
+            std::array<rank_type, __rank_> __permute{_Pos...};
+            for (rank_type __i = __rank_ - 1; __i > 0; __i--) {
+              for (rank_type __r = 0; __r < __i; __r++)
+                if (static_cast<index_type>(__strides[__permute[__r]]) >
+                    static_cast<index_type>(__strides[__permute[__r + 1]])) {
+                  swap(__permute[__r], __permute[__r + 1]);
+                } else {
+                  // if two strides are the same then one of the extents must be 1 or 0
+                  // both could be, but you can't have one larger than 1 come first
+                  if ((static_cast<index_type>(__strides[__permute[__r]]) ==
+                       static_cast<index_type>(__strides[__permute[__r + 1]])) &&
+                      (__ext.extent(__permute[__r]) > 1))
+                    swap(__permute[__r], __permute[__r + 1]);
+                }
+            }
+            // check that this permutations represents a growing set
+            for (rank_type __i = 1; __i < __rank_; __i++)
+              if (static_cast<index_type>(__strides[__permute[__i]]) <
+                  static_cast<index_type>(__strides[__permute[__i - 1]]) * __extents_.extent(__permute[__i - 1]))
+                return false;
+            return true;
+          }(make_index_sequence<__rank_>())),
+          "layout_stride::mapping ctor: the provided extents and strides lead to a non-unique mapping");
+    }
+  }
+
+  template <class _OtherIndexType>
+    requires(is_convertible_v<const _OtherIndexType&, index_type> &&
+             is_nothrow_constructible_v<index_type, const _OtherIndexType&>)
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping(const extents_type& __ext,
+                                          const array<_OtherIndexType, __rank_>& __strides) noexcept
+      : mapping(__ext, span(__strides)){};
+
+  template <class _StridedLayoutMapping>
+    requires(__mdspan_detail::__layout_mapping_alike<_StridedLayoutMapping> &&
+             is_constructible_v<extents_type, typename _StridedLayoutMapping::extents_type> &&
+             _StridedLayoutMapping::is_always_unique() && _StridedLayoutMapping::is_always_strided())
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(
+      !(is_convertible_v<typename _StridedLayoutMapping::extents_type, extents_type> &&
+        (__mdspan_detail::__is_mapping_of<layout_left, _StridedLayoutMapping> ||
+         __mdspan_detail::__is_mapping_of<layout_right, _StridedLayoutMapping> ||
+         __mdspan_detail::__is_mapping_of<layout_stride, _StridedLayoutMapping>)))
+      mapping(const _StridedLayoutMapping& __other) noexcept
+      : __extents_(__other.extents()), __strides_([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+          // stride() only compiles for rank > 0
+          if constexpr (__rank_ > 0) {
+            return __mdspan_detail::__possibly_empty_array<index_type, __rank_>{
+                static_cast<index_type>(__other.stride(_Pos))...};
+          } else {
+            return __mdspan_detail::__possibly_empty_array<index_type, 0>{};
+          }
+        }(make_index_sequence<__rank_>())) {
+    // stride() only compiles for rank > 0
+    if constexpr (__rank_ > 0) {
+      _LIBCPP_ASSERT(
+          ([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+            return ((static_cast<index_type>(__other.stride(_Pos)) > static_cast<index_type>(0)) && ... && true);
+          }(make_index_sequence<__rank_>())),
+          "layout_stride::mapping converting ctor: all strides must be greater than 0");
+    }
+    _LIBCPP_ASSERT(
+        __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
+        "layout_stride::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+    _LIBCPP_ASSERT(([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+                     return static_cast<index_type>(0) == static_cast<index_type>(__other((_Pos ? 0 : 0)...));
+                   }(make_index_sequence<__rank_>())),
+                   "layout_stride::mapping converting ctor: base offset of mapping must be zero.");
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping& operator=(const mapping&) noexcept = default;
+
+  // [mdspan.layout.stride.obs], observers
+  _LIBCPP_HIDE_FROM_ABI constexpr const extents_type& extents() const noexcept { return __extents_; }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr array<index_type, __rank_> strides() const noexcept {
+    return [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+      return array<index_type, __rank_>{__strides_[_Pos]...};
+    }(make_index_sequence<__rank_>());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr index_type required_span_size() const noexcept {
+    if constexpr (__rank_ == 0) {
+      return static_cast<index_type>(1);
+    } else {
+      return [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+        if ((__extents_.extent(_Pos) * ... * 1) == 0)
+          return static_cast<index_type>(0);
+        else
+          return static_cast<index_type>(
+              static_cast<index_type>(1) +
+              (((__extents_.extent(_Pos) - static_cast<index_type>(1)) * __strides_[_Pos]) + ... +
+               static_cast<index_type>(0)));
+      }(make_index_sequence<__rank_>());
+    }
+  }
+
+  template <class... _Indices>
+    requires((sizeof...(_Indices) == __rank_) && (is_convertible_v<_Indices, index_type> && ...) &&
+             (is_nothrow_constructible_v<index_type, _Indices> && ...))
+  _LIBCPP_HIDE_FROM_ABI constexpr index_type operator()(_Indices... __idx) const noexcept {
+    // Mappings are generally meant to be used for accessing allocations and are meant to guarantee to never
+    // return a value exceeding required_span_size(), which is used to know how large an allocation one needs
+    // Thus, this is a canonical point in multi-dimensional data structures to make invalid element access checks
+    // However, mdspan does check this on its own, so for now we avoid double checking in hardened mode
+    _LIBCPP_ASSERT(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
+                   "layout_stride::mapping: out of bounds indexing");
+    return [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+      return ((static_cast<index_type>(__idx) * __strides_[_Pos]) + ... + index_type(0));
+    }(make_index_sequence<sizeof...(_Indices)>());
+  }
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_unique() noexcept { return true; }
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_exhaustive() noexcept { return false; }
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_always_strided() noexcept { return true; }
+
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_unique() noexcept { return true; }
+  // The answer of this function is fairly complex in the case where one or more
+  // extents are zero.
+  // Technically its meaningless to query is_exhaustive() in that case, but unfortunately
+  // the way the standard defines this function, we can't give a simple true or false then.
+  _LIBCPP_HIDE_FROM_ABI constexpr bool is_exhaustive() const noexcept {
+    if constexpr (__rank_ == 0)
+      return true;
+    else {
+      index_type __span_size = required_span_size();
+      if (__span_size == static_cast<index_type>(0)) {
+        if constexpr (__rank_ == 1)
+          return true;
+        else {
+          rank_type __r_largest = 0;
+          for (rank_type __r = 1; __r < __rank_; __r++)
+            if (__strides_[__r] > __strides_[__r_largest])
+              __r_largest = __r;
+          for (rank_type __r = 0; __r < __rank_; __r++)
+            if (__extents_.extent(__r) == 0 && __r != __r_largest)
+              return false;
+          return true;
+        }
+      } else {
+        return required_span_size() == [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+          return (__extents_.extent(_Pos) * ... * static_cast<index_type>(1));
+        }(make_index_sequence<__rank_>());
+      }
+    }
+  }
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_strided() noexcept { return true; }
+
+  _LIBCPP_HIDE_FROM_ABI constexpr index_type stride(rank_type __r) const noexcept
+    requires(__rank_ > 0)
+  {
+    _LIBCPP_ASSERT(__r < __rank_, "layout_stride::mapping::stride(): invalid rank index");
+    return __strides_[__r];
+  }
+
+  template <class _OtherMapping>
+    requires(__mdspan_detail::__layout_mapping_alike<_OtherMapping> &&
+             (_OtherMapping::extents_type::rank() == __rank_) && _OtherMapping::is_always_strided())
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const mapping& __lhs, const _OtherMapping& __rhs) noexcept {
+    if (([&]<size_t... _Pos>(index_sequence<_Pos...>) {
+          return static_cast<index_type>(__rhs((_Pos ? 0 : 0)...));
+        }(make_index_sequence<__rank_>())) != static_cast<index_type>(0))
+      return false;
+    if constexpr (__rank_ == 0)
+      return true;
+    else {
+      return __lhs.extents() == __rhs.extents() && [&]<size_t... _Pos>(index_sequence<_Pos...>) {
+        // avoid warning when comparing signed and unsigner integers and pick the wider of two types
+        using _CommonType = common_type_t<index_type, typename _OtherMapping::index_type>;
+        return ((static_cast<_CommonType>(__lhs.stride(_Pos)) == static_cast<_CommonType>(__rhs.stride(_Pos))) && ... &&
+                true);
+      }(make_index_sequence<__rank_>());
+    }
+  }
+
+private:
+  _LIBCPP_NO_UNIQUE_ADDRESS extents_type __extents_{};
+  _LIBCPP_NO_UNIQUE_ADDRESS __mdspan_detail::__possibly_empty_array<index_type, __rank_> __strides_{};
+};
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___MDSPAN_LAYOUT_STRIDE_H

libcxx/include/mdspan

public:
     template<class OtherExtents>
       friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;
 
   private:
     extents_type extents_{};    // exposition only
   };
 }
 
+// layout_stride synopsis
+
+namespace std {
+  template<class Extents>
+  class layout_stride::mapping {
+  public:
+    using extents_type = Extents;
+    using index_type = typename extents_type::index_type;
+    using size_type = typename extents_type::size_type;
+    using rank_type = typename extents_type::rank_type;
+    using layout_type = layout_stride;
+
+  private:
+    static constexpr rank_type rank_ = extents_type::rank();    // exposition only
+
+  public:
+    // [mdspan.layout.stride.cons], constructors
+    constexpr mapping() noexcept;
+    constexpr mapping(const mapping&) noexcept = default;
+    template<class OtherIndexType>
+      constexpr mapping(const extents_type&, span<OtherIndexType, rank_>) noexcept;
+    template<class OtherIndexType>
+      constexpr mapping(const extents_type&, const array<OtherIndexType, rank_>&) noexcept;
+
+    template<class StridedLayoutMapping>
+      constexpr explicit(see below) mapping(const StridedLayoutMapping&) noexcept;
+
+    constexpr mapping& operator=(const mapping&) noexcept = default;
+
+    // [mdspan.layout.stride.obs], observers
+    constexpr const extents_type& extents() const noexcept { return extents_; }
+    constexpr array<index_type, rank_> strides() const noexcept { return strides_; }
+
+    constexpr index_type required_span_size() const noexcept;
+
+    template<class... Indices>
+      constexpr index_type operator()(Indices...) const noexcept;
+
+    static constexpr bool is_always_unique() noexcept { return true; }
+    static constexpr bool is_always_exhaustive() noexcept { return false; }
+    static constexpr bool is_always_strided() noexcept { return true; }
+
+    static constexpr bool is_unique() noexcept { return true; }
+    constexpr bool is_exhaustive() const noexcept;
+    static constexpr bool is_strided() noexcept { return true; }
+
+    constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; }
+
+    template<class OtherMapping>
+      friend constexpr bool operator==(const mapping&, const OtherMapping&) noexcept;
+
+  private:
+    extents_type extents_{};                    // exposition only
+    array<index_type, rank_> strides_{};        // exposition only
+  };
+}
+
 // default_accessor synopsis
 
 namespace std {
   template<class ElementType>
   struct default_accessor {
     using offset_policy = default_accessor;
     using element_type = ElementType;
     using reference = ElementType&;

 #define _LIBCPP_MDSPAN
 
 #include <__config>
 #include <__fwd/mdspan.h>
 #include <__mdspan/default_accessor.h>
 #include <__mdspan/extents.h>
 #include <__mdspan/layout_left.h>
 #include <__mdspan/layout_right.h>
+#include <__mdspan/layout_stride.h>
 #include <__mdspan/mdspan.h>
+#include <version>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 #endif // _LIBCPP_MDSPAN

libcxx/include/version

 # define __cpp_lib_constexpr_memory                     202202L
 # define __cpp_lib_constexpr_typeinfo                   202106L
 # define __cpp_lib_expected                             202211L
 # define __cpp_lib_format_ranges                        202207L
 // # define __cpp_lib_formatters                           202302L
 # define __cpp_lib_forward_like                         202207L
 # define __cpp_lib_invoke_r                             202106L
 # define __cpp_lib_is_scoped_enum                       202011L
-// # define __cpp_lib_mdspan                               202207L
+# define __cpp_lib_mdspan                               202207L
 // # define __cpp_lib_move_only_function                   202110L
 # undef  __cpp_lib_optional
 # define __cpp_lib_optional                             202110L
 // # define __cpp_lib_out_ptr                              202106L
 // # define __cpp_lib_print                                202207L
 # define __cpp_lib_ranges_as_rvalue                     202207L
 // # define __cpp_lib_ranges_chunk                         202202L
 // # define __cpp_lib_ranges_chunk_by                      202202L

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.conversion.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: libcpp-hardening-mode=unchecked
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class StridedLayoutMapping>
+//   constexpr explicit(see below)
+//     mapping(const StridedLayoutMapping& other) noexcept;
+//
+// Constraints:
+//   - layout-mapping-alike<StridedLayoutMapping> is satisfied.
+//   - is_constructible_v<extents_type, typename StridedLayoutMapping::extents_type> is true.
+//   - StridedLayoutMapping::is_always_unique() is true.
+//   - StridedLayoutMapping::is_always_strided() is true.
+//
+// Preconditions:
+//   - StridedLayoutMapping meets the layout mapping requirements ([mdspan.layout.policy.reqmts]),
+//   - other.stride(r) > 0 is true for every rank index r of extents(),
+//   - other.required_span_size() is representable as a value of type index_type ([basic.fundamental]), and
+//   - OFFSET(other) == 0 is true.
+//
+// Effects: Direct-non-list-initializes extents_ with other.extents(), and for all d in the range [0, rank_),
+//          direct-non-list-initializes strides_[d] with other.stride(d).
+//
+// Remarks: The expression inside explicit is equivalent to:
+//   - !(is_convertible_v<typename StridedLayoutMapping::extents_type, extents_type> &&
+//       (is-mapping-of<layout_left, LayoutStrideMapping> ||
+//        is-mapping-of<layout_right, LayoutStrideMapping> ||
+//        is-mapping-of<layout_stride, LayoutStrideMapping>))
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+#include "../../../../../std/containers/views/mdspan/CustomTestLayouts.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+
+  // working case
+  {
+    std::extents<int, D, D> arg_exts{100, 5};
+    std::layout_stride::mapping<std::extents<int, D, D>> arg(arg_exts, std::array<int, 2>{1,100});
+    [[maybe_unused]] std::layout_stride::mapping<std::extents<size_t, D, 5>> m(arg); // should work
+  }
+  // mismatch of static extent
+  {
+    std::extents<int, D, D> arg_exts{100, 5};
+    std::layout_stride::mapping<std::extents<int, D, D>> arg(arg_exts, std::array<int, 2>{1,100});
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_stride::mapping<std::extents<int, D, 3>> m(arg); }()),
+                               "extents construction: mismatch of provided arguments with static extents.");
+  }
+  // non-representability of extents itself
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_stride::mapping<std::extents<char, D>> m(
+                                 std::layout_stride::mapping<std::extents<int, D>>(std::extents<int, D>(500), std::array<int,1>{1})); }()),
+                               "extents ctor: arguments must be representable as index_type and nonnegative");
+  }
+  // all strides must be larger than zero
+  {
+    always_convertible_layout::mapping<std::dextents<int, 2>> offset_map(std::dextents<int ,2>{10,10},100,-1);
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::layout_stride::template mapping<std::extents<char, D, D>> m(offset_map); }()),
+        "layout_stride::mapping converting ctor: all strides must be greater than 0");
+  }
+  // required_span_size not representable, while individual extents are
+  {
+    std::extents<int, D, D> arg_exts{100, 5};
+    std::layout_stride::mapping<std::extents<int, D, D>> arg(arg_exts, std::array<int, 2>{1,100});
+    // check extents would be constructible
+    [[maybe_unused]] std::extents<char, D, 5> e(arg_exts);
+    // but the product is not, so we can't use it for layout_stride
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::layout_stride::template mapping<std::extents<char, D, 5>> m(arg); }()),
+        "layout_stride::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+  }
+  // base offset must be 0 (i.e. mapping(0,...,0)==0) for a strided layout with positiv strides
+  {
+    always_convertible_layout::mapping<std::dextents<int, 2>> offset_map(std::dextents<int ,2>{10,10},3);
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::layout_stride::template mapping<std::extents<char, D, D>> m(offset_map); }()),
+        "layout_stride::mapping converting ctor: base offset of mapping must be zero.");
+  }
+  return 0;
+}

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_array.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: libcpp-hardening-mode=unchecked
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class OtherIndexType>
+//   constexpr mapping(const extents_type& e, const array<OtherIndexType, rank_>& s) noexcept;
+//
+// Constraints:
+//   - is_convertible_v<const OtherIndexType&, index_type> is true, and
+//   - is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.
+//
+// Preconditions:
+//   - s[i] > 0 is true for all i in the range [0, rank_).
+//   - REQUIRED-SPAN-SIZE(e, s) is representable as a value of type index_type ([basic.fundamental]).
+//   - If rank_ is greater than 0, then there exists a permutation P of the integers in the range [0, rank_),
+//     such that s[pi] >= s[pi_1] * e.extent(pi_1) is true for all i in the range [1, rank_), where
+//     pi is the ith element of P.
+//     [Note 1: For layout_stride, this condition is necessary and sufficient for is_unique() to be true. end note]
+//
+// Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with as_const(s[d]).
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+
+  // value out of range
+  {
+    // the extents are representable but the product with strides is not, so we can't use it for layout_stride
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<char, D, 5>>
+            m(std::extents<char, D, 5>(20), std::array<int, 2>{20, 1});
+        }()),
+        "layout_stride::mapping ctor: required span size is not representable as index_type.");
+
+    // check that if we first overflow in strides conversion we also fail
+    assert(static_cast<unsigned char>(257u) == 1);
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<unsigned char, D, 5>>
+            m(std::extents<unsigned char, D, 5>(20), std::array<unsigned, 2>{257, 1});
+        }()),
+        "layout_stride::mapping ctor: required span size is not representable as index_type.");
+
+    // negative strides are not allowed, check with unsigned index_type so we make sure we catch that
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5>>
+            m(std::extents<unsigned, D, 5>(20), std::array<int, 2>{20, -1});
+        }()),
+        "layout_stride::mapping ctor: all strides must be greater than 0");
+    // zero strides are not allowed, check with unsigned index_type so we make sure we catch that
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5>>
+            m(std::extents<unsigned, D, 5>(20), std::array<unsigned, 2>{20, 0});
+        }()),
+        "layout_stride::mapping ctor: all strides must be greater than 0");
+    // strides must result in unique layout
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5, 7>>
+            m(std::extents<unsigned, D, 5, 7>(20), std::array<unsigned, 3>{4, 1, 200});
+        }()),
+        "layout_stride::mapping ctor: the provided extents and strides lead to a non-unique mapping");
+  }
+  return 0;
+}

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.ctor.extents_span.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: libcpp-hardening-mode=unchecked
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class OtherIndexType>
+//   constexpr mapping(const extents_type& e, span<OtherIndexType, rank_> s) noexcept;
+//
+// Constraints:
+//   - is_convertible_v<const OtherIndexType&, index_type> is true, and
+//   - is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.
+//
+// Preconditions:
+//   - s[i] > 0 is true for all i in the range [0, rank_).
+//   - REQUIRED-SPAN-SIZE(e, s) is representable as a value of type index_type ([basic.fundamental]).
+//   - If rank_ is greater than 0, then there exists a permutation P of the integers in the range [0, rank_),
+//     such that s[pi] >= s[pi_1] * e.extent(pi_1) is true for all i in the range [1, rank_), where
+//     pi is the ith element of P.
+//     [Note 1: For layout_stride, this condition is necessary and sufficient for is_unique() to be true. end note]
+//
+// Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_), direct-non-list-initializes strides_[d] with as_const(s[d]).
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+
+  // value out of range
+  {
+    // the extents are representable but the product with strides is not, so we can't use it for layout_stride
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::array<int, 2> strides{20, 1};
+          std::layout_stride::template mapping<std::extents<char, D, 5>>
+            m(std::extents<char, D, 5>(20), std::span(strides));
+        }()),
+        "layout_stride::mapping ctor: required span size is not representable as index_type.");
+
+    // check that if we first overflow in strides conversion we also fail
+    assert(static_cast<unsigned char>(257u) == 1);
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::array<unsigned, 2> strides{257, 1};
+          std::layout_stride::template mapping<std::extents<unsigned char, D, 5>>
+            m(std::extents<unsigned char, D, 5>(20), std::span(strides));
+        }()),
+        "layout_stride::mapping ctor: required span size is not representable as index_type.");
+
+    // negative strides are not allowed, check with unsigned index_type so we make sure we catch that
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::array<int, 2> strides{20, -1};
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5>>
+            m(std::extents<unsigned, D, 5>(20), std::span(strides));
+        }()),
+        "layout_stride::mapping ctor: all strides must be greater than 0");
+    // zero strides are not allowed, check with unsigned index_type so we make sure we catch that
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::array<unsigned, 2> strides{20, 0};
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5>>
+            m(std::extents<unsigned, D, 5>(20), std::span(strides));
+        }()),
+        "layout_stride::mapping ctor: all strides must be greater than 0");
+    // strides must result in unique layout
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] {
+          std::layout_stride::template mapping<std::extents<unsigned, D, 5, 7>>
+            m(std::extents<unsigned, D, 5, 7>(20), std::array<unsigned, 3>{4, 1, 200});
+        }()),
+        "layout_stride::mapping ctor: the provided extents and strides lead to a non-unique mapping");
+  }
+  return 0;
+}

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.index_operator.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: !libcpp-hardening-mode=debug
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// template<class... Indices>
+//   constexpr index_type operator()(Indices... i) const noexcept;
+//
+// Constraints:
+//   - sizeof...(Indices) == extents_type::rank() is true,
+//   - (is_convertible_v<Indices, index_type> && ...) is true, and
+//   - (is_nothrow_constructible_v<index_type, Indices> && ...) is true.
+//
+// Preconditions: extents_type::index-cast(i) is a multidimensional index in extents_ ([mdspan.overview]).
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  // value out of range
+  {
+    std::layout_stride::template mapping<std::extents<unsigned char, 5>> m;
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_stride::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_stride::template mapping<std::extents<signed char, 5>> m;
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_stride::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_stride::template mapping<std::dextents<unsigned char, 1>> m(std::dextents<unsigned char, 1>(5), std::array<int, 1>{1});
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_stride::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_stride::template mapping<std::dextents<signed char, 1>> m(std::dextents<signed char, 1>(5), std::array<int, 1>{1});
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_stride::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_stride::template mapping<std::dextents<int, 3>> m(std::dextents<int, 3>(5, 7, 9), std::array<int, 3>{1, 10, 100});
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, -1, -1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, 0, 0), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, -1, 0), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, 0, -1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 3, 3), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 7, 3), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 3, 9), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 7, 9), "layout_stride::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_stride::template mapping<std::dextents<unsigned, 3>> m(std::dextents<int, 3>(5, 7, 9), std::array<int, 3>{1, 10, 100});
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, -1, -1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, 0, 0), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, -1, 0), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, 0, -1), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 3, 3), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 7, 3), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 3, 9), "layout_stride::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 7, 9), "layout_stride::mapping: out of bounds indexing");
+  }
+  return 0;
+}

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.stride.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// UNSUPPORTED: libcpp-hardening-mode=unchecked
+// XFAIL: availability-verbose_abort-missing
+
+// <mdspan>
+
+// constexpr index_type stride(rank_type i) const noexcept { return strides_[i]; }
+
+
+// We intercept this inside layout_stride to give a consistent error message with
+// layout_left and layout_right, technically the precondition is coming from the
+// array access.
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  // value out of range
+  {
+    std::layout_stride::template mapping<std::dextents<int, 3>>
+      m(std::dextents<int, 3>(100, 100, 100), std::array<int, 3>{1,100,10000});
+
+    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "invalid rank index");
+  }
+  return 0;
+}

libcxx/test/libcxx/containers/views/mdspan/mdspan/assert.conversion.pass.cpp

 //   !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type>
 //   || !is_convertible_v<const OtherAccessor&, accessor_type>
 
 #include <array>
 #include <cassert>
 #include <mdspan>
 
 #include "check_assertion.h"
-#include "../../../../../std/containers/views/mdspan/mdspan/CustomTestLayouts.h"
+#include "../../../../../std/containers/views/mdspan/CustomTestLayouts.h"
 
 // We use a funky mapping in this test that doesn't check the dynamic/static extents mismatch itself
 int main(int, char**) {
   constexpr size_t D = std::dynamic_extent;
   std::array<float, 10> data;
   typename layout_wrapping_integral<4>::template mapping<std::dextents<int, 2>> src_map(
       std::dextents<int, 2>(5, 2), not_extents_constructible_tag());
   std::mdspan<float, std::dextents<int, 2>, layout_wrapping_integral<4>> arg(data.data(), src_map);

libcxx/test/libcxx/containers/views/mdspan/mdspan/assert.size.pass.cpp

 //
 // Returns: extents().fwd-prod-of-extents(rank()).
 
 #include <array>
 #include <cassert>
 #include <mdspan>
 
 #include "check_assertion.h"
-#include "../../../../../std/containers/views/mdspan/mdspan/CustomTestLayouts.h"
+#include "../../../../../std/containers/views/mdspan/CustomTestLayouts.h"
 
 // We use a funky mapping in this test where required_span_size is much smaller than the size of the index space
 int main(int, char**) {
   std::array<float, 10> data;
   // make sure we are not failing because of using index_type instead of size_type
   {
     typename layout_wrapping_integral<4>::template mapping<std::dextents<char, 2>> map(
       std::dextents<char, 2>(100, 2), not_extents_constructible_tag());

libcxx/test/libcxx/transitive_includes/cxx03.csv

 map utility
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory atomic
 memory compare
 memory concepts
 memory cstddef
 memory cstdint
 memory cstdlib
 memory cstring
 memory initializer_list

libcxx/test/libcxx/transitive_includes/cxx11.csv

 map utility
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory atomic
 memory compare
 memory concepts
 memory cstddef
 memory cstdint
 memory cstdlib
 memory cstring
 memory initializer_list

libcxx/test/libcxx/transitive_includes/cxx14.csv

 map utility
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory atomic
 memory compare
 memory concepts
 memory cstddef
 memory cstdint
 memory cstdlib
 memory cstring
 memory initializer_list

libcxx/test/libcxx/transitive_includes/cxx17.csv

 map utility
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory atomic
 memory compare
 memory concepts
 memory cstddef
 memory cstdint
 memory cstdlib
 memory cstring
 memory initializer_list

libcxx/test/libcxx/transitive_includes/cxx20.csv

 map utility
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory atomic
 memory compare
 memory concepts
 memory cstddef
 memory cstdint
 memory cstdlib
 memory cstring
 memory initializer_list

libcxx/test/libcxx/transitive_includes/cxx23.csv

 map tuple
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory compare
 memory cstddef
 memory cstdint
 memory cstring
 memory initializer_list
 memory iosfwd
 memory limits
 memory new

libcxx/test/libcxx/transitive_includes/cxx26.csv

 map tuple
 map version
 mdspan array
 mdspan cinttypes
 mdspan concepts
 mdspan cstddef
 mdspan limits
 mdspan span
+mdspan version
 memory compare
 memory cstddef
 memory cstdint
 memory cstring
 memory initializer_list
 memory iosfwd
 memory limits
 memory new

libcxx/test/std/containers/views/mdspan/CustomTestLayouts.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //                        Kokkos v. 4.0
 //       Copyright (2022) National Technology & Engineering
 //               Solutions of Sandia, LLC (NTESS).
 //
 // Under the terms of Contract DE-NA0003525 with NTESS,
 // the U.S. Government retains certain rights in this software.
 //
 //===---------------------------------------------------------------------===//
 
-#ifndef TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H
-#define TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H
+#ifndef TEST_STD_CONTAINERS_VIEWS_MDSPAN_CUSTOM_TEST_LAYOUTS_H
+#define TEST_STD_CONTAINERS_VIEWS_MDSPAN_CUSTOM_TEST_LAYOUTS_H
 
 #include <algorithm>
 #include <array>
 #include <cassert>
 #include <cinttypes>
 #include <concepts>
 #include <cstddef>
 #include <limits>

 template <size_t Wraps, class Extents>
 constexpr auto construct_mapping(layout_wrapping_integral<Wraps>, Extents exts) {
   return typename layout_wrapping_integral<Wraps>::template mapping<Extents>(exts, not_extents_constructible_tag{});
 }
 
 
 // This layout does not check convertibility of extents for its conversion ctor
 // Allows triggering mdspan's ctor static assertion on convertibility of extents
+// It also allows for negative strides and offsets via runtime arguments
 class always_convertible_layout {
 public:
   template <class Extents>
   class mapping;
 };
 
 template <class Extents>
 class always_convertible_layout::mapping {
for (rank_type r = 1; r < extents_type::rank(); r++) {
       if (overflowed)
         return false;
     }
     return true;
   }
 
 public:
   constexpr mapping() noexcept = delete;
-  constexpr mapping(const mapping& other) noexcept : extents_(other.extents()){};
-  constexpr mapping(const extents_type& ext) noexcept : extents_(ext){};
+  constexpr mapping(const mapping& other) noexcept : extents_(other.extents_), offset_(other.offset_), scaling_(other.scaling_){};
+  constexpr mapping(const extents_type& ext, index_type offset = 0, index_type scaling = 1) noexcept
+    : extents_(ext), offset_(offset), scaling_(scaling){};
 
   template <class OtherExtents>
   constexpr mapping(const mapping<OtherExtents>& other) noexcept {
     if constexpr (extents_type::rank() == OtherExtents::rank()) {
       std::array<index_type, extents_type::rank_dynamic()> dyn_extents;
       rank_type count = 0;
       for (rank_type r = 0; r < extents_type::rank(); r++) {
         if (extents_type::static_extent(r) == std::dynamic_extent) {
           dyn_extents[count++] = other.extents().extent(r);
         }
       }
       extents_ = extents_type(dyn_extents);
     } else {
       extents_ = extents_type();
     }
+    offset_ = other.offset_;
+    scaling_ = other.scaling_;
   }
 
   constexpr mapping& operator=(const mapping& other) noexcept {
     extents_ = other.extents_;
+    offset_ = other.offset_;
+    scaling_ = other.scaling_;
     return *this;
   };
 
   constexpr const extents_type& extents() const noexcept { return extents_; }
 
   constexpr index_type required_span_size() const noexcept {
     index_type size = 1;
     for (size_t r = 0; r < extents_type::rank(); r++)
       size *= extents_.extent(r);
-    return size;
+    return std::max(size*scaling_ + offset_, offset_);
   }
 
   template <std::integral... Indices>
     requires((sizeof...(Indices) == extents_type::rank()) && (std::is_convertible_v<Indices, index_type> && ...) &&
              (std::is_nothrow_constructible_v<index_type, Indices> && ...))
   constexpr index_type operator()(Indices... idx) const noexcept {
     std::array<index_type, extents_type::rank()> idx_a{static_cast<index_type>(static_cast<index_type>(idx))...};
-    return [&]<size_t... Pos>(std::index_sequence<Pos...>) {
+    return offset_ + scaling_ * ([&]<size_t... Pos>(std::index_sequence<Pos...>) {
       index_type res = 0;
       ((res = idx_a[extents_type::rank() - 1 - Pos] + extents_.extent(extents_type::rank() - 1 - Pos) * res), ...);
       return res;
-    }(std::make_index_sequence<sizeof...(Indices)>());
+    }(std::make_index_sequence<sizeof...(Indices)>()));
   }
 
-  static constexpr bool is_always_unique() noexcept { return false; }
+  static constexpr bool is_always_unique() noexcept { return true; }
   static constexpr bool is_always_exhaustive() noexcept { return true; }
-  static constexpr bool is_always_strided() noexcept { return false; }
+  static constexpr bool is_always_strided() noexcept { return true; }
 
   static constexpr bool is_unique() noexcept { return true; }
   static constexpr bool is_exhaustive() noexcept { return true; }
   static constexpr bool is_strided() noexcept { return true; }
 
   constexpr index_type stride(rank_type r) const noexcept
     requires(extents_type::rank() > 0)
   {
     index_type s = 1;
-    for (rank_type i = extents_type::rank() - 1; i > r; i--)
+    for (rank_type i = 0; i < r; i++)
       s *= extents_.extent(i);
-    return s;
+    return s*scaling_;
   }
 
   template <class OtherExtents>
     requires(OtherExtents::rank() == extents_type::rank())
   friend constexpr bool operator==(const mapping& lhs, const mapping<OtherExtents>& rhs) noexcept {
-    return lhs.extents() == rhs.extents();
+    return lhs.extents() == rhs.extents() && lhs.offset_ == rhs.offset && lhs.scaling_ == rhs.scaling_;
   }
 
   friend constexpr void swap(mapping& x, mapping& y) noexcept {
     swap(x.extents_, y.extents_);
     if !consteval {
       swap_counter()++;
     }
   }
 
   static int& swap_counter() {
     static int value = 0;
     return value;
   }
 
 private:
+  template<class>
+  friend class mapping;
+
   extents_type extents_{};
+  index_type offset_{};
+  index_type scaling_{};
 };
-#endif                     // TEST_STD_CONTAINERS_VIEWS_MDSPAN_MDSPAN_CUSTOM_TEST_LAYOUTS_H
+#endif                     // TEST_STD_CONTAINERS_VIEWS_MDSPAN_CUSTOM_TEST_LAYOUTS_H

libcxx/test/std/containers/views/mdspan/layout_stride/comparison.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherMapping>
+//   friend constexpr bool operator==(const mapping& x, const OtherMapping& y) noexcept;
+//
+// Constraints:
+//   - layout-mapping-alike<OtherMapping> is satisfied.
+//   - rank_ == OtherMapping::extents_type::rank() is true.
+//   - OtherMapping::is_always_strided() is true.
+//
+// Preconditions: OtherMapping meets the layout mapping requirements ([mdspan.layout.policy.reqmts]).
+//
+// Returns: true if x.extents() == y.extents() is true, OFFSET(y) == 0 is true, and each of x.stride(r) == y.stride(r) is true for r in the range [0, x.extents().rank()). Otherwise, false.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+#include "../CustomTestLayouts.h"
+
+template <class To, class From>
+constexpr void test_comparison(bool equal, To dest_exts, From src_exts,
+                                           std::array<int, To::rank()> dest_strides,
+                                           std::array<int, From::rank()> src_strides) {
+  std::layout_stride::mapping<To> dest(dest_exts, dest_strides);
+  std::layout_stride::mapping<From> src(src_exts, src_strides);
+  ASSERT_NOEXCEPT(dest == src);
+  assert((dest == src) == equal);
+  assert((dest != src) == !equal);
+}
+
+struct X {
+  constexpr bool does_not_match() { return true; }
+};
+
+constexpr X compare_layout_mappings(...) { return {}; }
+
+template <class E1, class E2>
+constexpr auto compare_layout_mappings(E1 e1, std::array<int, E1::rank()> s1, E2 e2, std::array<int, E2::rank()> s2)
+    -> decltype(std::layout_stride::mapping<E1>(e1, s1) == std::layout_stride::mapping<E2>(e2, s2)) {
+  return true;
+}
+
+template <class T1, class T2>
+constexpr void test_comparison_different_rank() {
+  constexpr size_t D = std::dynamic_extent;
+
+  using arr_0 = std::array<int, 0>;
+  using arr_1 = std::array<int, 1>;
+  using arr_2 = std::array<int, 2>;
+
+  // sanity check same rank
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), arr_1{1}, std::extents<T2, D>(5), arr_1{1}));
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), arr_1{1}, std::extents<T2, D>(5), arr_1{1}));
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), arr_1{1}, std::extents<T2, 5>(), arr_1{1}));
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), arr_1{1}, std::extents<T2, 5>(), arr_1{1}));
+
+  // not equality comparable when rank is not the same
+  static_assert(compare_layout_mappings(std::extents<T1>(), arr_0{}, std::extents<T2, D>(1), arr_1{1}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1>(), arr_0{}, std::extents<T2, 1>(), arr_1{1}).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D>(1), arr_1{1}, std::extents<T2>(), arr_0{}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 1>(), arr_1{1}, std::extents<T2>(), arr_0{}).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), arr_1{1}, std::extents<T2, D, D>(5, 5), arr_2{1, 5}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), arr_1{1}, std::extents<T2, 5, D>(5), arr_2{1, 5}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), arr_1{1}, std::extents<T2, 5, 1>(), arr_2{1, 5}).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D, D>(5, 5), arr_2{1, 5}, std::extents<T2, D>(5), arr_1{1}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5, D>(5), arr_2{1, 5}, std::extents<T2, D>(5), arr_1{1}).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5, 5>(), arr_2{1, 5}, std::extents<T2, 5>(), arr_1{1}).does_not_match());
+}
+
+template <class T1, class T2>
+constexpr void test_comparison_same_rank() {
+  constexpr size_t D = std::dynamic_extent;
+
+  test_comparison(true, std::extents<T1>(), std::extents<T2>(),
+                        std::array<int, 0>{}, std::array<int, 0>{});
+
+  test_comparison(true, std::extents<T1, D>(5), std::extents<T2, D>(5),
+                        std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(true, std::extents<T1, 5>(), std::extents<T2, D>(5),
+                        std::array<int, 1>{3}, std::array<int, 1>{3});
+  test_comparison(true, std::extents<T1, D>(5), std::extents<T2, 5>(),
+                        std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(true, std::extents<T1, 5>(), std::extents< T2, 5>(),
+                        std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, 5>(), std::extents<T2, D>(5),
+                         std::array<int, 1>{2}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, D>(5), std::extents<T2, D>(5),
+                         std::array<int, 1>{2}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, D>(5), std::extents<T2, D>(7),
+                         std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, 5>(), std::extents<T2, D>(7),
+                         std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, D>(5), std::extents<T2, 7>(),
+                         std::array<int, 1>{1}, std::array<int, 1>{1});
+  test_comparison(false, std::extents<T1, 5>(), std::extents<T2, 7>(),
+                         std::array<int, 1>{1}, std::array<int, 1>{1});
+
+  test_comparison(true, std::extents<T1, D, D, D, D, D>(5, 6, 7, 8, 9), std::extents<T2, D, D, D, D, D>(5, 6, 7, 8, 9),
+                        std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(true, std::extents<T1, D, 6, D, 8, D>(5, 7, 9), std::extents<T2, 5, D, D, 8, 9>(6, 7),
+                        std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(true, std::extents<T1, 5, 6, 7, 8, 9>(5, 6, 7, 8, 9), std::extents<T2, 5, 6, 7, 8, 9>(),
+                        std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(false, std::extents<T1, 5, 6, 7, 8, 9>(5, 6, 7, 8, 9), std::extents<T2, 5, 6, 7, 8, 9>(),
+                         std::array<int, 5>{2, 20, 200, 20000, 2000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(
+      false, std::extents<T1, D, D, D, D, D>(5, 6, 7, 8, 9), std::extents<T2, D, D, D, D, D>(5, 6, 3, 8, 9),
+                         std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(false, std::extents<T1, D, 6, D, 8, D>(5, 7, 9), std::extents<T2, 5, D, D, 3, 9>(6, 7),
+                         std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+  test_comparison(false, std::extents<T1, 5, 6, 7, 8, 9>(5, 6, 7, 8, 9), std::extents<T2, 5, 6, 7, 3, 9>(),
+                         std::array<int, 5>{2, 20, 200, 2000, 20000}, std::array<int, 5>{2, 20, 200, 2000, 20000});
+}
+
+template <class OtherLayout, class E1, class E2, class ... OtherArgs>
+constexpr void test_comparison_with(bool expect_equal, E1 e1, std::array<typename E1::index_type, E1::rank()> strides, E2 e2, OtherArgs ... other_args){
+  typename std::layout_stride::template mapping<E1> map(e1, strides);
+  typename OtherLayout::template mapping<E2> other_map(e2, other_args...);
+
+  assert((map == other_map) == expect_equal);
+}
+
+template <class OtherLayout>
+constexpr void test_comparison_with() {
+  constexpr size_t D = std::dynamic_extent;
+  bool is_left_based = std::is_same_v<OtherLayout, std::layout_left> ||
+                       std::is_same_v<OtherLayout, always_convertible_layout>;
+  test_comparison_with<OtherLayout>(true, std::extents<int>(), std::array<int, 0>{}, std::extents<unsigned>());
+  test_comparison_with<OtherLayout>(true, std::extents<int, 5>(), std::array<int, 1>{1}, std::extents<unsigned, 5>());
+  test_comparison_with<OtherLayout>(true, std::extents<int, D>(5), std::array<int, 1>{1}, std::extents<unsigned, 5>());
+  test_comparison_with<OtherLayout>(false, std::extents<int, D>(5), std::array<int, 1>{2}, std::extents<unsigned, 5>());
+  test_comparison_with<OtherLayout>(is_left_based, std::extents<int, D, D>(5, 7), std::array<int, 2>{1, 5}, std::extents<unsigned, D, D>(5, 7));
+  test_comparison_with<OtherLayout>(!is_left_based, std::extents<int, D, D>(5, 7), std::array<int, 2>{7, 1}, std::extents<unsigned, D, D>(5, 7));
+  test_comparison_with<OtherLayout>(false, std::extents<int, D, D>(5, 7), std::array<int, 2>{8, 1}, std::extents<unsigned, D, D>(5, 7));
+
+  if constexpr (std::is_same_v<OtherLayout, always_convertible_layout>) {
+    // test layout with strides not equal to product of extents
+    test_comparison_with<OtherLayout>(true, std::extents<int, D, D>(5, 7), std::array<int, 2>{2, 10}, std::extents<unsigned, D, D>(5, 7), 0, 2);
+    // make sure that offset != 0 results in false
+    test_comparison_with<OtherLayout>(false, std::extents<int, D, D>(5, 7), std::array<int, 2>{2, 10}, std::extents<unsigned, D, D>(5, 7), 1, 2);
+  }
+}
+
+template <class T1, class T2>
+constexpr void test_comparison_index_type() {
+  test_comparison_same_rank<T1, T2>();
+  test_comparison_different_rank<T1, T2>();
+  test_comparison_with<std::layout_right>();
+  test_comparison_with<std::layout_left>();
+  test_comparison_with<always_convertible_layout>();
+}
+
+constexpr bool test() {
+  test_comparison_index_type<int, int>();
+  test_comparison_index_type<int, size_t>();
+  test_comparison_index_type<size_t, int>();
+  test_comparison_index_type<size_t, long>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.default.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// Test default construction:
+//
+// constexpr mapping() noexcept;
+//
+//
+// Preconditions: layout_right::mapping<extents_type>().required_span_size() is representable as a value of type index_type ([basic.fundamental]).
+//
+// Effects: Direct-non-list-initializes extents_ with extents_type(), and for all d in the range [0, rank_),
+//          direct-non-list-initializes strides_[d] with layout_right::mapping<extents_type>().stride(d).
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_construction() {
+  using M = std::layout_stride::mapping<E>;
+  ASSERT_NOEXCEPT(M{});
+  M m;
+  E e;
+
+  // check correct extents are returned
+  ASSERT_NOEXCEPT(m.extents());
+  assert(m.extents() == e);
+
+  // check required_span_size()
+  typename E::index_type expected_size = 1;
+  for (typename E::rank_type r = 0; r < E::rank(); r++)
+    expected_size *= e.extent(r);
+  assert(m.required_span_size() == expected_size);
+
+  // check strides: node stride function is constrained on rank>0, e.extent(r) is not
+  auto strides = m.strides();
+  ASSERT_NOEXCEPT(m.strides());
+  if constexpr (E::rank() > 0) {
+    std::layout_right::mapping<E> m_right;
+    for (typename E::rank_type r = 0; r < E::rank(); r++) {
+      assert(m.stride(r) == m_right.stride(r));
+      assert(strides[r] == m.stride(r));
+    }
+  }
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_construction<std::extents<int>>();
+  test_construction<std::extents<unsigned, D>>();
+  test_construction<std::extents<unsigned, 7>>();
+  test_construction<std::extents<unsigned, 0>>();
+  test_construction<std::extents<unsigned, 7, 8>>();
+  test_construction<std::extents<int64_t, D, 8, D, D>>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.extents_array.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherIndexType>
+//  constexpr mapping(const extents_type& e, array<OtherIndexType, rank_> s) noexcept;
+//
+// Constraints:
+//    - is_convertible_v<const OtherIndexType&, index_type> is true, and
+//    - is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.
+//
+// Preconditions:
+//    - s[i] > 0 is true for all i in the range [0, rank_).
+//    - REQUIRED-SPAN-SIZE(e, s) is representable as a value of type index_type ([basic.fundamental]).
+//    - If rank_ is greater than 0, then there exists a permutation P of the integers in the range [0, rank_),
+//      such that s[pi] >= s[pi_1] * e.extent(pi_1) is true for all i in the range [1, rank_), where pi is the ith element of P.
+//     Note 1: For layout_stride, this condition is necessary and sufficient for is_unique() to be true.
+//
+// Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_),
+//         direct-non-list-initializes strides_[d] with as_const(s[d]).
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+#include "../ConvertibleToIntegral.h"
+
+template <class E, class S>
+constexpr void test_construction(E e, S s) {
+  using M = std::layout_stride::mapping<E>;
+  ASSERT_NOEXCEPT(M{e, s});
+  M m(e, s);
+
+  // check correct extents are returned
+  ASSERT_NOEXCEPT(m.extents());
+  assert(m.extents() == e);
+
+  // check required_span_size()
+  typename E::index_type expected_size = 1;
+  for (typename E::rank_type r = 0; r < E::rank(); r++) {
+    if (e.extent(r) == 0) { expected_size = 0; break; }
+    expected_size += (e.extent(r) - 1) * static_cast<typename E::index_type>(s[r]);
+  }
+  assert(m.required_span_size() == expected_size);
+
+  // check strides: node stride function is constrained on rank>0, e.extent(r) is not
+  auto strides = m.strides();
+  ASSERT_NOEXCEPT(m.strides());
+  if constexpr (E::rank() > 0) {
+    for (typename E::rank_type r = 0; r < E::rank(); r++) {
+      assert(m.stride(r) == static_cast<typename E::index_type>(s[r]));
+      assert(strides[r] == m.stride(r));
+    }
+  }
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  {
+    std::array<int, 0> s{};
+    test_construction(std::extents<int>(), s);
+  }
+  {
+    std::array<int, 1> s{1};
+    test_construction(std::extents<unsigned, D>(7), s);
+  }
+  {
+    std::array<int, 1> s{1};
+    test_construction(std::extents<unsigned, D>(0), s);
+  }
+  {
+    std::array<int, 1> s{2};
+    test_construction(std::extents<unsigned, 7>(), s);
+  }
+  {
+    std::array<IntType, 1> s{1};
+    test_construction(std::extents<int, D>(7), s);
+  }
+  {
+    std::array<int, 2> s{3, 30};
+    test_construction(std::extents<unsigned, 7, 8>(), s);
+  }
+  {
+    std::array<int, 4> s{20, 2, 200, 2000};
+    test_construction(std::extents<int64_t, D, 8, D, D>(7, 9, 10), s);
+    test_construction(std::extents<int64_t, D, 8, D, D>(0, 9, 10), s);
+    test_construction(std::extents<int64_t, D, 8, D, D>(0, 8, 0), s);
+  }
+  {
+    std::array<int, 4> s{200, 20, 20, 2000};
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 8, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 8, 0, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 8, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 8, 1, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 1, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 0, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 1, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 0, 9), s);
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 1, 9), s);
+  }
+
+  {
+    using mapping_t = std::layout_stride::mapping<std::dextents<unsigned, 2>>;
+    // wrong strides size
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::array<int, 3>>);
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::array<int, 1>>);
+    // wrong extents rank
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 3>, std::array<int, 2>>);
+    // none-convertible strides
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::array<IntType, 2>>);
+  }
+  {
+    // not no-throw constructible index_type from stride
+    using mapping_t = std::layout_stride::mapping<std::dextents<unsigned char, 2>>;
+    static_assert(std::is_convertible_v<IntType, unsigned char>);
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::array<IntType, 2>>);
+  }
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.extents_span.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class OtherIndexType>
+//  constexpr mapping(const extents_type& e, span<OtherIndexType, rank_> s) noexcept;
+//
+// Constraints:
+//    - is_convertible_v<const OtherIndexType&, index_type> is true, and
+//    - is_nothrow_constructible_v<index_type, const OtherIndexType&> is true.
+//
+// Preconditions:
+//    - s[i] > 0 is true for all i in the range [0, rank_).
+//    - REQUIRED-SPAN-SIZE(e, s) is representable as a value of type index_type ([basic.fundamental]).
+//    - If rank_ is greater than 0, then there exists a permutation P of the integers in the range [0, rank_),
+//      such that s[pi] >= s[pi_1] * e.extent(pi_1) is true for all i in the range [1, rank_), where pi is the ith element of P.
+//     Note 1: For layout_stride, this condition is necessary and sufficient for is_unique() to be true.
+//
+// Effects: Direct-non-list-initializes extents_ with e, and for all d in the range [0, rank_),
+//         direct-non-list-initializes strides_[d] with as_const(s[d]).
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+#include "../ConvertibleToIntegral.h"
+
+template <class E, class S>
+constexpr void test_construction(E e, S s) {
+  using M = std::layout_stride::mapping<E>;
+  ASSERT_NOEXCEPT(M{e, s});
+  M m(e, s);
+
+  // check correct extents are returned
+  ASSERT_NOEXCEPT(m.extents());
+  assert(m.extents() == e);
+
+  // check required_span_size()
+  typename E::index_type expected_size = 1;
+  for (typename E::rank_type r = 0; r < E::rank(); r++) {
+    if (e.extent(r) == 0) { expected_size = 0; break; }
+    expected_size += (e.extent(r) - 1) * static_cast<typename E::index_type>(s[r]);
+  }
+  assert(m.required_span_size() == expected_size);
+
+  // check strides: node stride function is constrained on rank>0, e.extent(r) is not
+  auto strides = m.strides();
+  ASSERT_NOEXCEPT(m.strides());
+  if constexpr (E::rank() > 0) {
+    for (typename E::rank_type r = 0; r < E::rank(); r++) {
+      assert(m.stride(r) == static_cast<typename E::index_type>(s[r]));
+      assert(strides[r] == m.stride(r));
+    }
+  }
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  {
+    std::array<int, 0> s{};
+    test_construction(std::extents<int>(), std::span(s));
+  }
+  {
+    std::array<int, 1> s{1};
+    test_construction(std::extents<unsigned, D>(7), std::span(s));
+  }
+  {
+    std::array<int, 1> s{1};
+    test_construction(std::extents<unsigned, D>(0), std::span(s));
+  }
+  {
+    std::array<int, 1> s{2};
+    test_construction(std::extents<unsigned, 7>(), std::span(s));
+  }
+  {
+    std::array<IntType, 1> s{1};
+    test_construction(std::extents<int, D>(7), std::span(s));
+  }
+  {
+    std::array<int, 2> s{3, 30};
+    test_construction(std::extents<unsigned, 7, 8>(), std::span(s));
+  }
+  {
+    std::array<int, 4> s{20, 2, 200, 2000};
+    test_construction(std::extents<int64_t, D, 8, D, D>(7, 9, 10), std::span(s));
+  }
+  {
+    std::array<int, 4> s{20, 2, 200, 2000};
+    test_construction(std::extents<int64_t, D, 8, D, D>(7, 0, 10), std::span(s));
+    test_construction(std::extents<int64_t, D, 8, D, D>(0, 9, 10), std::span(s));
+    test_construction(std::extents<int64_t, D, 8, D, D>(0, 8, 0), std::span(s));
+  }
+  {
+    std::array<int, 4> s{200, 20, 20, 2000};
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 8, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 8, 0, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 8, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 8, 1, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 1, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 0, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 1, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 1, 0, 9), std::span(s));
+    test_construction(std::extents<int64_t, D, D, D, D>(7, 0, 1, 9), std::span(s));
+  }
+
+  {
+    using mapping_t = std::layout_stride::mapping<std::dextents<unsigned, 2>>;
+    // wrong strides size
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::span<int, 3>>);
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::span<int, 1>>);
+    // wrong extents rank
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 3>, std::span<int, 2>>);
+    // none-convertible strides
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::span<IntType, 2>>);
+  }
+  {
+    // not no-throw constructible index_type from stride
+    using mapping_t = std::layout_stride::mapping<std::dextents<unsigned char, 2>>;
+    static_assert(std::is_convertible_v<IntType, unsigned char>);
+    static_assert(!std::is_constructible_v<mapping_t, std::dextents<int, 2>, std::span<IntType, 2>>);
+  }
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/ctor.strided_mapping.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class StridedLayoutMapping>
+//   constexpr explicit(see below)
+//     mapping(const StridedLayoutMapping& other) noexcept;
+//
+// Constraints:
+//   - layout-mapping-alike<StridedLayoutMapping> is satisfied.
+//   - is_constructible_v<extents_type, typename StridedLayoutMapping::extents_type> is true.
+//   - StridedLayoutMapping::is_always_unique() is true.
+//   - StridedLayoutMapping::is_always_strided() is true.
+//
+// Preconditions:
+//   - StridedLayoutMapping meets the layout mapping requirements ([mdspan.layout.policy.reqmts]),
+//   - other.stride(r) > 0 is true for every rank index r of extents(),
+//   - other.required_span_size() is representable as a value of type index_type ([basic.fundamental]), and
+//   - OFFSET(other) == 0 is true.
+//
+// Effects: Direct-non-list-initializes extents_ with other.extents(), and for all d in the range [0, rank_),
+//          direct-non-list-initializes strides_[d] with other.stride(d).
+//
+// Remarks: The expression inside explicit is equivalent to:
+//   - !(is_convertible_v<typename StridedLayoutMapping::extents_type, extents_type> &&
+//       (is-mapping-of<layout_left, LayoutStrideMapping> ||
+//        is-mapping-of<layout_right, LayoutStrideMapping> ||
+//        is-mapping-of<layout_stride, LayoutStrideMapping>))
+
+#include <mdspan>
+#include <type_traits>
+#include <cassert>
+#include <limits>
+
+#include "test_macros.h"
+
+#include "../CustomTestLayouts.h"
+
+template <class To, class From>
+constexpr void test_implicit_conversion(To dest, From src) {
+  assert(dest == src);
+}
+
+template <bool implicit, class FromL, class ToE, class FromE>
+constexpr void test_conversion(FromE src_exts) {
+  using To   = std::layout_stride::mapping<ToE>;
+  using From = FromL::template mapping<FromE>;
+
+  From src([&]() {
+    if constexpr (std::is_same_v<FromL, std::layout_stride>) {
+      // just construct some strides which aren't layout_left/layout_right
+      std::array<size_t, FromE::rank()> strides;
+      size_t stride = 2;
+      for(size_t r=0; r<FromE::rank(); r++) {
+        strides[r] = stride;
+        stride *= src_exts.extent(r);
+      }
+      return From(src_exts, strides);
+    } else {
+      return From(src_exts);
+    }
+  }());
+
+  ASSERT_NOEXCEPT(To(src));
+  To dest(src);
+
+  assert(dest == src);
+  if constexpr (implicit) {
+    dest = src;
+    assert(dest == src);
+    test_implicit_conversion<To, From>(src, src);
+  } else {
+    assert((!std::is_convertible_v<From, To>));
+  }
+}
+
+template <class FromL, class T1, class T2>
+constexpr void test_conversion() {
+  constexpr size_t D = std::dynamic_extent;
+  constexpr bool idx_convertible =
+      static_cast<size_t>(std::numeric_limits<T1>::max()) >= static_cast<size_t>(std::numeric_limits<T2>::max());
+  constexpr bool l_convertible =
+      std::is_same_v<FromL, std::layout_right> || std::is_same_v<FromL, std::layout_left> || std::is_same_v<FromL, std::layout_stride>;
+  constexpr bool idx_l_convertible = idx_convertible && l_convertible;
+
+  // clang-format off
+  // adding extents convertibility expectation
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1>>(std::extents<T2>());
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, D>>(std::extents<T2, D>(5));
+  test_conversion<idx_l_convertible && false, FromL, std::extents<T1, 5>>(std::extents<T2, D>(5));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, 5>>(std::extents<T2, 5>());
+  test_conversion<idx_l_convertible && false, FromL, std::extents<T1, 5, D>>(std::extents<T2, D, D>(5, 5));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, D, D>>(std::extents<T2, D, D>(5, 5));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, D, D>>(std::extents<T2, D, 7>(5));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, 5, 7>>(std::extents<T2, 5, 7>());
+  test_conversion<idx_l_convertible && false, FromL, std::extents<T1, 5, D, 8, D, D>>(std::extents<T2, D, D, 8, 9, 1>(5, 7));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, D, D, D, D, D>>(
+                                                     std::extents<T2, D, D, D, D, D>(5, 7, 8, 9, 1));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, D, D, 8, 9, D>>(std::extents<T2, D, 7, 8, 9, 1>(5));
+  test_conversion<idx_l_convertible && true,  FromL, std::extents<T1, 5, 7, 8, 9, 1>>(std::extents<T2, 5, 7, 8, 9, 1>());
+  // clang-format on
+}
+
+template <class IdxT, size_t... Extents>
+using ToM = typename std::layout_stride::template mapping<std::extents<IdxT, Extents...>>;
+
+template <class FromL, class IdxT, size_t... Extents>
+using FromM = typename FromL::template mapping<std::extents<IdxT, Extents...>>;
+
+template<class FromL>
+constexpr void test_no_implicit_conversion() {
+  constexpr size_t D = std::dynamic_extent;
+
+  // Sanity check that one static to dynamic conversion works
+  static_assert(std::is_constructible_v<ToM<int, D>, FromM<FromL, int, 5>>);
+  static_assert(std::is_convertible_v<FromM<FromL, int, 5>, ToM<int, D>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<ToM<int, 5>, FromM<FromL, int, D>>);
+  static_assert(!std::is_convertible_v<FromM<FromL, int, D>, ToM<int, 5>>);
+
+  // Sanity check that one static to dynamic conversion works
+  static_assert(std::is_constructible_v<ToM<int, D, 7>, FromM<FromL, int, 5, 7>>);
+  static_assert(std::is_convertible_v<FromM<FromL, int, 5, 7>, ToM<int, D, 7>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<ToM<int, 5, 7>, FromM<FromL, int, D, 7>>);
+  static_assert(!std::is_convertible_v<FromM<FromL, int, D, 7>, ToM<int, 5, 7>>);
+
+  // Sanity check that smaller index_type to larger index_type conversion works
+  static_assert(std::is_constructible_v<ToM<size_t, 5>, FromM<FromL, int, 5>>);
+  static_assert(std::is_convertible_v<FromM<FromL, int, 5>, ToM<size_t, 5>>);
+
+  // Check that larger index_type to smaller index_type conversion works explicitly only
+  static_assert(std::is_constructible_v<ToM<int, 5>, FromM<FromL, size_t, 5>>);
+  static_assert(!std::is_convertible_v<FromM<FromL, size_t, 5>, ToM<int, 5>>);
+}
+
+template<class FromL>
+constexpr void test_rank_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<ToM<int, D>, FromM<FromL, int>>);
+  static_assert(!std::is_constructible_v<ToM<int>, FromM<FromL, int, D, D>>);
+  static_assert(!std::is_constructible_v<ToM<int, D>, FromM<FromL, int, D, D>>);
+  static_assert(!std::is_constructible_v<ToM<int, D, D, D>, FromM<FromL, int, D, D>>);
+}
+
+template<class FromL>
+constexpr void test_static_extent_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<ToM<int, D, 5>, FromM<FromL, int, D, 4>>);
+  static_assert(!std::is_constructible_v<ToM<int, 5>, FromM<FromL, int, 4>>);
+  static_assert(!std::is_constructible_v<ToM<int, 5, D>, FromM<FromL, int, 4, D>>);
+}
+
+template<class FromL>
+constexpr void test_layout() {
+  test_conversion<FromL, int, int>();
+  test_conversion<FromL, int, size_t>();
+  test_conversion<FromL, size_t, int>();
+  test_conversion<FromL, size_t, long>();
+  // the implicit convertibility test doesn't apply to non std::layouts
+  if constexpr (!std::is_same_v<FromL, always_convertible_layout>)
+    test_no_implicit_conversion<FromL>();
+  test_rank_mismatch<FromL>();
+  test_static_extent_mismatch<FromL>();
+}
+
+constexpr bool test() {
+  test_layout<std::layout_right>();
+  test_layout<std::layout_left>();
+  test_layout<std::layout_stride>();
+  test_layout<always_convertible_layout>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/extents.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class Extents>
+// class layout_stride::mapping;
+
+// If Extents is not a specialization of extents, then the program is
+// ill-formed.
+
+// Mandates: If Extents::rank_dynamic() == 0 is true, then the size of the
+// multidimensional index space Extents() is representable as a value of type
+// typename Extents::index_type.
+
+#include <mdspan>
+
+void not_extents() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}layout_stride::mapping template argument must be a specialization of extents}}
+  [[maybe_unused]] std::layout_stride::mapping<void> mapping;
+}
+
+void representable() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}layout_stride::mapping product of static extents must be representable as index_type.}}
+  [[maybe_unused]] std::layout_stride::mapping<std::extents<char, 20, 20>> mapping;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/index_operator.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// Test default iteration:
+//
+// template<class... Indices>
+//   constexpr index_type operator()(Indices...) const noexcept;
+//
+// Constraints:
+//   * sizeof...(Indices) == extents_type::rank() is true,
+//   * (is_convertible_v<Indices, index_type> && ...) is true, and
+//   * (is_nothrow_constructible_v<index_type, Indices> && ...) is true.
+//
+// Preconditions:
+//   * extents_type::index-cast(i) is a multidimensional index in extents_.
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+#include "../ConvertibleToIntegral.h"
+
+template <class Mapping, class... Indices>
+concept operator_constraints = requires(Mapping m, Indices... idxs) {
+  { std::is_same_v<decltype(m(idxs...)), typename Mapping::index_type> };
+};
+
+template <class Mapping, class... Indices>
+  requires(operator_constraints<Mapping, Indices...>)
+constexpr bool check_operator_constraints(Mapping m, Indices... idxs) {
+  (void)m(idxs...);
+  return true;
+}
+
+template <class Mapping, class... Indices>
+constexpr bool check_operator_constraints(Mapping, Indices...) {
+  return false;
+}
+
+template <class M, class... Args>
+constexpr void iterate_stride(M m, const std::array<int, M::extents_type::rank()>& strides, Args... args) {
+  constexpr int r = static_cast<int>(M::extents_type::rank()) - 1 - static_cast<int>(sizeof...(Args));
+  if constexpr (-1 == r) {
+    ASSERT_NOEXCEPT(m(args...));
+    size_t expected_val = [&]<size_t... Pos>(std::index_sequence<Pos...>) {
+      return ((args * strides[Pos]) + ... + 0);
+    }(std::make_index_sequence<M::extents_type::rank()>());
+    assert(expected_val == static_cast<size_t>(m(args...)));
+  } else {
+    for (typename M::index_type i = 0; i < m.extents().extent(r); i++) {
+      iterate_stride(m, strides, i, args...);
+    }
+  }
+}
+
+template <class E, class... Args>
+constexpr void test_iteration(std::array<int, E::rank()> strides, Args... args) {
+  using M = std::layout_stride::mapping<E>;
+  M m(E(args...), strides);
+
+  iterate_stride(m, strides);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_iteration<std::extents<int>>(std::array<int, 0>{});
+  test_iteration<std::extents<unsigned, D>>(std::array<int, 1>{2}, 1);
+  test_iteration<std::extents<unsigned, D>>(std::array<int, 1>{3}, 7);
+  test_iteration<std::extents<unsigned, 7>>(std::array<int, 1>{4});
+  test_iteration<std::extents<unsigned, 7, 8>>(std::array<int, 2>{25, 3});
+  test_iteration<std::extents<char, D, D, D, D>>(std::array<int, 4>{1, 1, 1, 1}, 1, 1, 1, 1);
+
+  // Check operator constraint for number of arguments
+  static_assert(check_operator_constraints(
+      std::layout_stride::mapping<std::extents<int, D>>(std::extents<int, D>(1), std::array{1}), 0));
+  static_assert(!check_operator_constraints(
+      std::layout_stride::mapping<std::extents<int, D>>(std::extents<int, D>(1), std::array{1}), 0, 0));
+
+  // Check operator constraint for convertibility of arguments to index_type
+  static_assert(check_operator_constraints(
+      std::layout_stride::mapping<std::extents<int, D>>(std::extents<int, D>(1), std::array{1}), IntType(0)));
+  static_assert(!check_operator_constraints(
+      std::layout_stride::mapping<std::extents<unsigned, D>>(std::extents<unsigned, D>(1), std::array{1}), IntType(0)));
+
+  // Check operator constraint for no-throw-constructibility of index_type from arguments
+  static_assert(!check_operator_constraints(
+      std::layout_stride::mapping<std::extents<unsigned char, D>>(std::extents<unsigned char, D>(1), std::array{1}),
+      IntType(0)));
+
+  return true;
+}
+
+constexpr bool test_large() {
+  constexpr size_t D = std::dynamic_extent;
+  test_iteration<std::extents<int64_t, D, 8, D, D>>(std::array<int, 4>{2000, 2, 20, 200}, 7, 9, 10);
+  test_iteration<std::extents<int64_t, D, 8, 1, D>>(std::array<int, 4>{2000, 20, 20, 200}, 7, 10);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+
+  // The large test iterates over ~10k loop indices.
+  // With assertions enabled this triggered the maximum default limit
+  // for steps in consteval expressions. Assertions roughly double the
+  // total number of instructions, so this was already close to the maximum.
+  test_large();
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/is_exhaustive_corner_case.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// constexpr bool is_exhaustive() const noexcept;
+//
+// Returns:
+//   - true if rank_ is 0.
+//   - Otherwise, true if there is a permutation P of the integers in the range [0, rank_) such that
+//     stride(p0) equals 1, and stride(pi) equals stride(pi_1) * extents().extent(pi_1) for i in the
+//     range [1, rank_), where pi is the ith element of P.
+//   - Otherwise, false.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()> strides, bool exhaustive) {
+  using M = std::layout_stride::template mapping<E>;
+  M m(ext, strides);
+  assert(m.is_exhaustive() == exhaustive);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_layout_mapping_stride(std::extents<int, 0>(), std::array<int,1>{1}, true);
+  test_layout_mapping_stride(std::extents<int, D>(0), std::array<int,1>{3}, true);
+  test_layout_mapping_stride(std::extents<int, 0, 3>(), std::array<int,2>{6,2}, true);
+  test_layout_mapping_stride(std::extents<int, D, D>(3, 0), std::array<int,2>{6,2}, false);
+  test_layout_mapping_stride(std::extents<int, D, D>(0, 0), std::array<int,2>{6,2}, false);
+  test_layout_mapping_stride(std::extents<int, D, D, D, D>(3,3,0,3),std::array<int,4>{3,1,27,9}, true);
+  test_layout_mapping_stride(std::extents<int, D, D, D, D>(0,3,3,3),std::array<int,4>{3,1,27,9}, false);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/properties.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// namespace std {
+//   template<class Extents>
+//   class layout_stride::mapping {
+//
+//     ...
+//     static constexpr bool is_always_unique() noexcept { return true; }
+//     static constexpr bool is_always_exhaustive() noexcept { return false; }
+//     static constexpr bool is_always_strided() noexcept { return true; }
+//
+//     static constexpr bool is_unique() noexcept { return true; }
+//     static constexpr bool is_exhaustive() noexcept;
+//     static constexpr bool is_strided() noexcept { return true; }
+//     ...
+//   };
+// }
+//
+//
+// layout_stride::mapping<E> is a trivially copyable type that models regular for each E.
+//
+// constexpr bool is_exhaustive() const noexcept;
+//
+// Returns:
+//   - true if rank_ is 0.
+//   - Otherwise, true if there is a permutation P of the integers in the range [0, rank_) such that
+//     stride(p0) equals 1, and stride(pi) equals stride(pi_1) * extents().extent(pi_1) for i in the
+//     range [1, rank_), where pi is the ith element of P.
+//   - Otherwise, false.
+
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_layout_mapping_stride(E ext, std::array<typename E::index_type, E::rank()> strides, bool exhaustive) {
+  using M = std::layout_stride::template mapping<E>;
+  M m(ext, strides);
+  assert(M::is_unique() == true);
+  assert(m.is_exhaustive() == exhaustive);
+  assert(M::is_strided() == true);
+  assert(M::is_always_unique() == true);
+  assert(M::is_always_exhaustive() == false);
+  assert(M::is_always_strided() == true);
+  ASSERT_NOEXCEPT(std::declval<M>().is_unique());
+  ASSERT_NOEXCEPT(std::declval<M>().is_exhaustive());
+  ASSERT_NOEXCEPT(std::declval<M>().is_strided());
+  ASSERT_NOEXCEPT(M::is_always_unique());
+  ASSERT_NOEXCEPT(M::is_always_exhaustive());
+  ASSERT_NOEXCEPT(M::is_always_strided());
+  static_assert(std::is_trivially_copyable_v<M>);
+  static_assert(std::regular<M>);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_layout_mapping_stride(std::extents<int>(), std::array<int,0>{}, true);
+  test_layout_mapping_stride(std::extents<char, 4, 5>(), std::array<char,2>{1,4}, true);
+  test_layout_mapping_stride(std::extents<char, 4, 5>(), std::array<char,2>{1,5}, false);
+  test_layout_mapping_stride(std::extents<unsigned, D, 4>(7), std::array<unsigned,2>{20,2}, false);
+  test_layout_mapping_stride(std::extents<size_t, D, D, D, D>(3,3,3,3),std::array<size_t,4>{3,1,9,27}, true);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/required_span_size.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// Let REQUIRED-SPAN-SIZE(e, strides) be:
+//    - 1, if e.rank() == 0 is true,
+//    - otherwise 0, if the size of the multidimensional index space e is 0,
+//    - otherwise 1 plus the sum of products of (e.extent(r) - 1) and strides[r] for all r in the range [0, e.rank()).
+
+// constexpr index_type required_span_size() const noexcept;
+//
+//   Returns: REQUIRED-SPAN-SIZE(extents(), strides_).
+
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_required_span_size(E e, std::array<int, E::rank()> strides, typename E::index_type expected_size) {
+  using M = std::layout_stride::mapping<E>;
+  const M m(e, strides);
+
+  ASSERT_NOEXCEPT(m.required_span_size());
+  assert(m.required_span_size() == expected_size);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_required_span_size(std::extents<int>(), std::array<int, 0>{}, 1);
+  test_required_span_size(std::extents<unsigned, D>(0), std::array<int, 1>{5}, 0);
+  test_required_span_size(std::extents<unsigned, D>(1), std::array<int, 1>{5}, 1);
+  test_required_span_size(std::extents<unsigned, D>(7), std::array<int, 1>{5}, 31);
+  test_required_span_size(std::extents<unsigned, 7>(), std::array<int, 1>{5}, 31);
+  test_required_span_size(std::extents<unsigned, 7, 8>(), std::array<int, 2>{20, 2}, 135);
+  test_required_span_size(std::extents<int64_t, D, 8, D, D>(7, 9, 10), std::array<int, 4>{1, 7, 7*8, 7*8*9}, 5040);
+  test_required_span_size(std::extents<int64_t, 1, 8, D, D>(9, 10), std::array<int, 4>{1, 7, 7*8, 7*8*9}, 5034);
+  test_required_span_size(std::extents<int64_t, 1, 0, D, D>(9, 10), std::array<int, 4>{1, 7, 7*8, 7*8*9}, 0);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/static_requirements.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// A type M meets the layout mapping requirements if
+//    - M models copyable and equality_comparable,
+//    - is_nothrow_move_constructible_v<M> is true,
+//    - is_nothrow_move_assignable_v<M> is true,
+//    - is_nothrow_swappable_v<M> is true, and
+//
+// the following types and expressions are well-formed and have the specified semantics.
+//
+//  typename M::extents_type
+//    Result: A type that is a specialization of extents.
+//
+//  typename M::index_type
+//    Result: typename M::extents_type::index_type.
+//
+//  typename M::rank_type
+//    Result: typename M::extents_type::rank_type.
+//
+//  typename M::layout_type
+//    Result: A type MP that meets the layout mapping policy requirements ([mdspan.layout.policy.reqmts]) and for which is-mapping-of<MP, M> is true.
+//
+//  m.extents()
+//    Result: const typename M::extents_type&
+//
+//  m(i...)
+//    Result: typename M::index_type
+//    Returns: A nonnegative integer less than numeric_limits<typename M::index_type>::max() and less than or equal to numeric_limits<size_t>::max().
+//
+//  m(i...) == m(static_cast<typename M::index_type>(i)...)
+//    Result: bool
+//    Returns: true
+//
+//  m.required_span_size()
+//    Result: typename M::index_type
+//    Returns: If the size of the multidimensional index space m.extents() is 0, then 0, else 1 plus the maximum value of m(i...) for all i.
+//
+//  m.is_unique()
+//    Result: bool
+//    Returns: true only if for every i and j where (i != j || ...) is true, m(i...) != m(j...) is true.
+//
+//  m.is_exhaustive()
+//    Result: bool
+//    Returns: true only if for all k in the range [0, m.required_span_size()) there exists an i such that m(i...) equals k.
+//
+//  m.is_strided()
+//    Result: bool
+//    Returns: true only if for every rank index r of m.extents() there exists an integer
+//             sr such that, for all i where (i+dr) is a multidimensional index in m.extents() ([mdspan.overview]),
+//             m((i + dr)...) - m(i...) equals sr
+//
+//  m.stride(r)
+//    Preconditions: m.is_strided() is true.
+//    Result: typename M::index_type
+//    Returns: sr as defined in m.is_strided() above.
+//
+//  M::is_always_unique()
+//    Result: A constant expression ([expr.const]) of type bool.
+//    Returns: true only if m.is_unique() is true for all possible objects m of type M.
+//
+//  M::is_always_exhaustive()
+//    Result: A constant expression ([expr.const]) of type bool.
+//    Returns: true only if m.is_exhaustive() is true for all possible objects m of type M.
+//
+//  M::is_always_strided()
+//    Result: A constant expression ([expr.const]) of type bool.
+//    Returns: true only if m.is_strided() is true for all possible objects m of type M.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+// Common requirements of all layout mappings
+template <class M, size_t... Idxs>
+void test_mapping_requirements(std::index_sequence<Idxs...>) {
+  using E = typename M::extents_type;
+  static_assert(std::__mdspan_detail::__is_extents_v<E>);
+  static_assert(std::is_copy_constructible_v<M>);
+  static_assert(std::is_nothrow_move_constructible_v<M>);
+  static_assert(std::is_nothrow_move_assignable_v<M>);
+  static_assert(std::is_nothrow_swappable_v<M>);
+  ASSERT_SAME_TYPE(typename M::index_type, typename E::index_type);
+  ASSERT_SAME_TYPE(typename M::size_type, typename E::size_type);
+  ASSERT_SAME_TYPE(typename M::rank_type, typename E::rank_type);
+  ASSERT_SAME_TYPE(typename M::layout_type, std::layout_stride);
+  ASSERT_SAME_TYPE(typename M::layout_type::template mapping<E>, M);
+  static_assert(std::is_same_v<decltype(std::declval<M>().extents()), const E&>);
+  static_assert(std::is_same_v<decltype(std::declval<M>().strides()), std::array<typename M::index_type, E::rank()>>);
+  static_assert(std::is_same_v<decltype(std::declval<M>()(Idxs...)), typename M::index_type>);
+  static_assert(std::is_same_v<decltype(std::declval<M>().required_span_size()), typename M::index_type>);
+  static_assert(std::is_same_v<decltype(std::declval<M>().is_unique()), bool>);
+  static_assert(std::is_same_v<decltype(std::declval<M>().is_exhaustive()), bool>);
+  static_assert(std::is_same_v<decltype(std::declval<M>().is_strided()), bool>);
+  if constexpr (E::rank() > 0)
+    static_assert(std::is_same_v<decltype(std::declval<M>().stride(0)), typename M::index_type>);
+  static_assert(std::is_same_v<decltype(M::is_always_unique()), bool>);
+  static_assert(std::is_same_v<decltype(M::is_always_exhaustive()), bool>);
+  static_assert(std::is_same_v<decltype(M::is_always_strided()), bool>);
+}
+
+template <class L, class E>
+void test_layout_mapping_requirements() {
+  using M = typename L::template mapping<E>;
+  test_mapping_requirements<M>(std::make_index_sequence<E::rank()>());
+}
+
+template <class E>
+void test_layout_mapping_stride() {
+  test_layout_mapping_requirements<std::layout_stride, E>();
+}
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  test_layout_mapping_stride<std::extents<int>>();
+  test_layout_mapping_stride<std::extents<char, 4, 5>>();
+  test_layout_mapping_stride<std::extents<unsigned, D, 4>>();
+  test_layout_mapping_stride<std::extents<size_t, D, D, D, D>>();
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_stride/stride.pass.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// constexpr index_type stride(rank_type i) const noexcept;
+//
+//   Constraints: extents_type::rank() > 0 is true.
+//
+//   Preconditions: i < extents_type::rank() is true.
+//
+//   Returns: extents().rev-prod-of-extents(i).
+
+#include <mdspan>
+#include <cassert>
+#include <array>
+#include <cstdint>
+#include <cstdio>
+#include "test_macros.h"
+
+template <class E, class... Args>
+constexpr void test_stride(std::array<typename E::index_type, E::rank()> strides, Args... args) {
+  using M = std::layout_stride::mapping<E>;
+  M m(E(args...), strides);
+
+  ASSERT_NOEXCEPT(m.stride(0));
+  for (size_t r = 0; r < E::rank(); r++)
+    assert(strides[r] == m.stride(r));
+
+  ASSERT_NOEXCEPT(m.strides());
+  auto strides_out = m.strides();
+  static_assert(std::is_same_v<decltype(strides_out), std::array<typename E::index_type, E::rank()>>);
+  for (size_t r = 0; r < E::rank(); r++)
+    assert(strides[r] == strides_out[r]);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_stride<std::extents<unsigned, D>>(std::array<unsigned, 1>{1}, 7);
+  test_stride<std::extents<unsigned, 7>>(std::array<unsigned, 1>{1});
+  test_stride<std::extents<unsigned, 7, 8>>(std::array<unsigned, 2>{8, 1});
+  test_stride<std::extents<int64_t, D, 8, D, D>>(std::array<int64_t, 4>{720, 90, 10, 1}, 7, 9, 10);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/mdspan/assign.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   MDS m_org(handle, map, acc);
   MDS m(handle, map, acc);

libcxx/test/std/containers/views/mdspan/mdspan/conversion.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class ToMDS, class FromMDS>
 constexpr void test_implicit_conversion(ToMDS to_mds, FromMDS from_mds) {
   assert(to_mds.extents() == from_mds.extents());
   if constexpr (std::equality_comparable_with<typename ToMDS::data_handle_type, typename FromMDS::data_handle_type>)
     assert(to_mds.data_handle() == from_mds.data_handle());
   if constexpr (std::equality_comparable_with<typename ToMDS::mapping_type, typename FromMDS::mapping_type>)

libcxx/test/std/containers/views/mdspan/mdspan/conversion.verify.cpp

 //   - direct-non-list-initializes acc_ with other.acc_.
 //
 // Remarks: The expression inside explicit is equivalent to:
 //   !is_convertible_v<const OtherLayoutPolicy::template mapping<OtherExtents>&, mapping_type>
 //   || !is_convertible_v<const OtherAccessor&, accessor_type>
 
 #include <mdspan>
 #include "CustomTestAccessors.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 
 void cant_construct_data_handle_type() {
   int data;
   std::mdspan<int, std::extents<int>, std::layout_right, convertible_accessor_but_not_handle<int>> m_nc(&data);
   // expected-error-re@*:* {{{{.*}}no matching constructor for initialization of {{.*}} (aka 'not_const_convertible_handle<const int>')}}
   // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}mdspan: incompatible data_handle_type for mdspan construction}}
   [[maybe_unused]] std::
       mdspan<const int, std::extents<int>, std::layout_right, convertible_accessor_but_not_handle<const int>>

libcxx/test/std/containers/views/mdspan/mdspan/ctor.copy.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   MDS m_org(handle, map, acc);
   MDS m(m_org);

libcxx/test/std/containers/views/mdspan/mdspan/ctor.default.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <bool hc, bool mc, bool ac, class H, class M, class A>
 constexpr void test_mdspan_types(const H&, const M&, const A&) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   static_assert(hc == std::is_default_constructible_v<H>);
   static_assert(mc == std::is_default_constructible_v<M>);

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_array.pass.cpp

 #include <cassert>
 #include <mdspan>
 #include <type_traits>
 
 #include "test_macros.h"
 
 #include "../ConvertibleToIntegral.h"
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class Extents, size_t... Idxs>
 constexpr auto array_from_extents(const Extents& exts, std::index_sequence<Idxs...>) {
   return std::array<typename Extents::index_type, Extents::rank()>{exts.extent(Idxs)...};
 }
 
 template <class MDS, class Exts>

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_extents.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <bool mec, bool ac, class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A&) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   static_assert(mec == std::is_constructible_v<M, const typename M::extents_type&>);
   static_assert(ac == std::is_default_constructible_v<A>);

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_integers.pass.cpp

 #include <concepts>
 #include <cassert>
 #include <mdspan>
 #include <type_traits>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class MDS, class... Args>
 concept check_mdspan_ctor_implicit = requires(MDS m, Args... args) { m = {args...}; };
 
 template <bool mec, bool ac, class H, class M, class A, class... Idxs>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A&, Idxs... idxs) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_map.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <bool ac, class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A&) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   static_assert(ac == std::is_default_constructible_v<A>);
   if constexpr (ac) {

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_map_acc.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   if !consteval {
     move_counted_handle<typename MDS::element_type>::move_counter() = 0;

libcxx/test/std/containers/views/mdspan/mdspan/ctor.dh_span.pass.cpp

 #include <cassert>
 #include <mdspan>
 #include <type_traits>
 
 #include "test_macros.h"
 
 #include "../ConvertibleToIntegral.h"
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class Extents, size_t... Idxs>
 constexpr auto array_from_extents(const Extents& exts, std::index_sequence<Idxs...>) {
   return std::array<typename Extents::index_type, Extents::rank()>{exts.extent(Idxs)...};
 }
 
 template <class MDS, class Exts>

libcxx/test/std/containers/views/mdspan/mdspan/ctor.move.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   MDS m_org(handle, map, acc);
   MDS m(std::move(m_org));

libcxx/test/std/containers/views/mdspan/mdspan/deduction.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   // deduction from data_handle_type (including non-pointer), mapping and accessor
   ASSERT_SAME_TYPE(decltype(std::mdspan(handle, map, acc)), MDS);

libcxx/test/std/containers/views/mdspan/mdspan/index_operator.pass.cpp

 
 #include <mdspan>
 #include <cassert>
 #include <cstdint>
 
 #include "test_macros.h"
 
 #include "../ConvertibleToIntegral.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 
 // Clang 15 and 16 do not support argument packs as input to operator []
 #if defined(__clang_major__) && __clang_major__ < 17
 template <class MDS>
 constexpr auto& access(MDS mds) {
   return mds[];
 }
 template <class MDS>

libcxx/test/std/containers/views/mdspan/mdspan/move.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 #include "CustomTestAccessors.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
 
   MDS m_org(handle, map, acc);
   MDS m_copy(m_org);

libcxx/test/std/containers/views/mdspan/mdspan/properties.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 
 template <class H, class M, class A>
 constexpr void test_mdspan_types(const H& handle, const M& map, const A& acc) {
   using MDS = std::mdspan<typename A::element_type, typename M::extents_type, typename M::layout_type, A>;
   MDS m(handle, map, acc);
 
   // =====================================
   // Traits for every mdspan

libcxx/test/std/containers/views/mdspan/mdspan/swap.pass.cpp

 #include <mdspan>
 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 
 template <class MDS>
 constexpr void test_swap(MDS a, MDS b) {
   auto org_a = a;
   auto org_b = b;
   swap(a, b);
   assert(a.extents() == org_b.extents());
   assert(b.extents() == org_a.extents());

libcxx/test/std/containers/views/mdspan/mdspan/types.pass.cpp

 #include <type_traits>
 #include <concepts>
 #include <cassert>
 
 #include "test_macros.h"
 
 #include "../MinimalElementType.h"
 #include "CustomTestAccessors.h"
-#include "CustomTestLayouts.h"
+#include "../CustomTestLayouts.h"
 
 // Calculated expected size of an mdspan
 // Note this expectes that only default_accessor is empty
 template<class MDS>
 constexpr size_t expected_size() {
   size_t sizeof_dht = sizeof(typename MDS::data_handle_type);
   size_t result = sizeof_dht;
   if(MDS::rank_dynamic() > 0) {

libcxx/test/std/language.support/support.limits/support.limits.general/mdspan.version.compile.pass.cpp

 # endif
 
 # ifdef __cpp_lib_submdspan
 #   error "__cpp_lib_submdspan should not be defined before c++26"
 # endif
 
 #elif TEST_STD_VER == 23
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should be defined in c++23"
-#   endif
-#   if __cpp_lib_mdspan != 202207L
-#     error "__cpp_lib_mdspan should have the value 202207L in c++23"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should not be defined because it is unimplemented in libc++!"
-#   endif
-# endif
+# ifndef __cpp_lib_mdspan
+#   error "__cpp_lib_mdspan should be defined in c++23"
+# endif
+# if __cpp_lib_mdspan != 202207L
+#   error "__cpp_lib_mdspan should have the value 202207L in c++23"
+# endif
 
 # ifdef __cpp_lib_submdspan
 #   error "__cpp_lib_submdspan should not be defined before c++26"
 # endif
 
 #elif TEST_STD_VER > 23
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should be defined in c++26"
-#   endif
-#   if __cpp_lib_mdspan != 202207L
-#     error "__cpp_lib_mdspan should have the value 202207L in c++26"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should not be defined because it is unimplemented in libc++!"
-#   endif
-# endif
+# ifndef __cpp_lib_mdspan
+#   error "__cpp_lib_mdspan should be defined in c++26"
+# endif
+# if __cpp_lib_mdspan != 202207L
+#   error "__cpp_lib_mdspan should have the value 202207L in c++26"
+# endif
 
 # if !defined(_LIBCPP_VERSION)
 #   ifndef __cpp_lib_submdspan
 #     error "__cpp_lib_submdspan should be defined in c++26"
 #   endif
 #   if __cpp_lib_submdspan != 202306L
 #     error "__cpp_lib_submdspan should have the value 202306L in c++26"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_submdspan
 #     error "__cpp_lib_submdspan should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
 #endif // TEST_STD_VER > 23
 

libcxx/test/std/language.support/support.limits/support.limits.general/version.version.compile.pass.cpp

 #     error "__cpp_lib_math_special_functions should have the value 201603L in c++23"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_math_special_functions
 #     error "__cpp_lib_math_special_functions should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should be defined in c++23"
-#   endif
-#   if __cpp_lib_mdspan != 202207L
-#     error "__cpp_lib_mdspan should have the value 202207L in c++23"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should not be defined because it is unimplemented in libc++!"
-#   endif
-# endif
+# ifndef __cpp_lib_mdspan
+#   error "__cpp_lib_mdspan should be defined in c++23"
+# endif
+# if __cpp_lib_mdspan != 202207L
+#   error "__cpp_lib_mdspan should have the value 202207L in c++23"
+# endif
 
 # if !defined(_LIBCPP_AVAILABILITY_HAS_NO_PMR)
 #   ifndef __cpp_lib_memory_resource
 #     error "__cpp_lib_memory_resource should be defined in c++23"
 #   endif
 #   if __cpp_lib_memory_resource != 201603L
 #     error "__cpp_lib_memory_resource should have the value 201603L in c++23"
 #   endif

 #     error "__cpp_lib_math_special_functions should have the value 201603L in c++26"
 #   endif
 # else // _LIBCPP_VERSION
 #   ifdef __cpp_lib_math_special_functions
 #     error "__cpp_lib_math_special_functions should not be defined because it is unimplemented in libc++!"
 #   endif
 # endif
 
-# if !defined(_LIBCPP_VERSION)
-#   ifndef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should be defined in c++26"
-#   endif
-#   if __cpp_lib_mdspan != 202207L
-#     error "__cpp_lib_mdspan should have the value 202207L in c++26"
-#   endif
-# else // _LIBCPP_VERSION
-#   ifdef __cpp_lib_mdspan
-#     error "__cpp_lib_mdspan should not be defined because it is unimplemented in libc++!"
-#   endif
-# endif
+# ifndef __cpp_lib_mdspan
+#   error "__cpp_lib_mdspan should be defined in c++26"
+# endif
+# if __cpp_lib_mdspan != 202207L
+#   error "__cpp_lib_mdspan should have the value 202207L in c++26"
+# endif
 
 # if !defined(_LIBCPP_AVAILABILITY_HAS_NO_PMR)
 #   ifndef __cpp_lib_memory_resource
 #     error "__cpp_lib_memory_resource should be defined in c++26"
 #   endif
 #   if __cpp_lib_memory_resource != 201603L
 #     error "__cpp_lib_memory_resource should have the value 201603L in c++26"
 #   endif

libcxx/utils/generate_feature_test_macro_components.py

for x in [
             "values": {"c++17": 201603},
             "headers": ["cmath"],
             "unimplemented": True,
         },
         {
             "name": "__cpp_lib_mdspan",
             "values": {"c++23": 202207},
             "headers": ["mdspan"],
-            "unimplemented": True,
         },
         {
             "name": "__cpp_lib_memory_resource",
             "values": {"c++17": 201603},
             "headers": ["memory_resource"],
             "test_suite_guard": "!defined(_LIBCPP_AVAILABILITY_HAS_NO_PMR)",
             "libcxx_guard": "!defined(_LIBCPP_AVAILABILITY_HAS_NO_PMR)",
         },