[libc++] Adding mdspan layout_left

[crtrott]

Summary

This adds the layout_left layout mapping for mdspan, and its associated tests.
The tests are copy paste from layout_right, with more or less a search replace of left for right..

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

[ldionne]

I think the next step would be rebasing this properly on top of layouts_right so the diff is fixed, but this pretty much LGTM.

libcxx/include/__mdspan/layout_left.h

74

99

Here and probably elsewhere.

127

I think we should take a look at the codegen here for small but especially for large numbers of dimensions to make sure the compiler doesn't create an actual array.

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

34–35

Either address or remove.

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

43

I would probably add this and call it a day:

// we can't actually test the representability requirement on `required_span_size()` because it triggers the assertion above in
// the extents constructor

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

25

I don't think this is needed.

108–109

I would add !std::is_constructible_v<lr_mapping_t<int, 1, 1>, ll_mapping_t<int, 1, 1>>

126

Oops!

[crtrott]

Rebased on top of layout_right revision

[crtrott]

Address review comments.

[crtrott]

Fix test which exceeds default consteval step limit, and fix formatting of fwd file.

[crtrott]

Fix some copy paste name issues in required_span_size test
Fix whitespace in comments in tests

[crtrott]

Fix missing include for module build

[ldionne]

LGTM, I'll make a local check for the diff between the tests for layout_right and layout_left just in case.

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

25

You're not using any concepts, I think this can go.

[crtrott]

Remove unused include in test

[crtrott]

libcxx/include/__mdspan/layout_left.h

127

We decided to do this after the fact, lets us play on godbolt etc.

[crtrott]

Apply post-review changes from layout_right to layout_left

libcxx/docs/Status/Cxx23.rst

 
 .. csv-table::
    :file: Cxx23Papers.csv
    :header-rows: 1
    :widths: auto
 
 .. note::
 
-   .. [#note-P0009R18] P0009R18: ``extents``, ``dextents``, and ``layout_right`` are implemented.
+   .. [#note-P0009R18] P0009R18: ``extents``, ``dextents``, ``layout_left``, and ``layout_right`` 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] P1413R3: The formatter for ``std::thread::id`` is implemented.
       The formatter for ``stacktrace`` is not implemented, since ``stacktrace`` is
       not implemented yet.

libcxx/include/CMakeLists.txt

set(files
   __iterator/unreachable_sentinel.h
   __iterator/wrap_iter.h
   __locale
   __locale_dir/locale_base_api/bsd_locale_defaults.h
   __locale_dir/locale_base_api/bsd_locale_fallbacks.h
   __locale_dir/locale_base_api/locale_guard.h
   __mbstate_t.h
   __mdspan/extents.h
+  __mdspan/layout_left.h
   __mdspan/layout_right.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

 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if _LIBCPP_STD_VER >= 23
 
-/*
-// Will be implemented with follow on revision
 // Layout policy with a mapping which corresponds to FORTRAN-style array layouts
 struct layout_left {
   template<class Extents>
     class mapping;
 };
-*/
 
 // Layout policy with a mapping which corresponds to C-style array layouts
 struct layout_right {
   template <class Extents>
   class mapping;
 };
 
 /*

libcxx/include/__mdspan/layout_left.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_LEFT_H
+#define _LIBCPP___MDSPAN_LAYOUT_LEFT_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/integer_sequence.h>
+#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
+
+template <class _Extents>
+class layout_left::mapping {
+public:
+  static_assert(__mdspan_detail::__is_extents<_Extents>::value,
+                "layout_left::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_left;
+
+private:
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool __required_span_size_is_representable(const extents_type& __ext) {
+    if constexpr (extents_type::rank() == 0)
+      return true;
+
+    index_type __prod = __ext.extent(0);
+    for (rank_type __r = 1; __r < extents_type::rank(); __r++) {
+      bool __overflowed = __builtin_mul_overflow(__prod, __ext.extent(__r), &__prod);
+      if (__overflowed)
+        return false;
+    }
+    return true;
+  }
+
+  static_assert((extents_type::rank_dynamic() > 0) || __required_span_size_is_representable(extents_type()),
+                "layout_left::mapping product of static extents must be representable as index_type.");
+
+public:
+  // [mdspan.layout.left.cons], constructors
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping() noexcept               = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping(const mapping&) noexcept = default;
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping(const extents_type& __ext) noexcept : __extents_(__ext) {
+    _LIBCPP_ASSERT(__required_span_size_is_representable(__ext),
+                   "layout_left::mapping extents ctor: product of extents must be representable as index_type.");
+  }
+
+  template <class _OtherExtents>
+    requires(is_constructible_v<extents_type, _OtherExtents>)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
+      mapping(const mapping<_OtherExtents>& __other) noexcept
+      : __extents_(__other.extents()) {
+    _LIBCPP_ASSERT(
+        __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
+        "layout_left::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+  }
+
+  template <class _OtherExtents>
+    requires(is_constructible_v<extents_type, _OtherExtents> && _OtherExtents::rank() <= 1)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
+      mapping(const layout_right::mapping<_OtherExtents>& __other) noexcept
+      : __extents_(__other.extents()) {
+    _LIBCPP_ASSERT(
+        __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
+        "layout_left::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+  }
+
+// FIXME: add when we add other layouts
+#  if 0
+    template<class _OtherExtents>
+      constexpr explicit(extents_type::rank() > 0)
+        mapping(const layout_stride::mapping_<OtherExtents>&) noexcept;
+#  endif
+
+  _LIBCPP_HIDE_FROM_ABI constexpr mapping& operator=(const mapping&) noexcept = default;
+
+  // [mdspan.layout.left.obs], observers
+  _LIBCPP_HIDE_FROM_ABI constexpr const extents_type& extents() const noexcept { return __extents_; }
+
+  _LIBCPP_HIDE_FROM_ABI 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;
+  }
+
+  template <class... _Indices>
+    requires((sizeof...(_Indices) == extents_type::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 {
+    _LIBCPP_ASSERT(__mdspan_detail::__is_multidimensional_index_in(__extents_, __idx...),
+                   "layout_left::mapping: out of bounds indexing");
+    array<index_type, extents_type::rank()> __idx_a{static_cast<index_type>(__idx)...};
+    return [&]<size_t... _Pos>(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;
+    }(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 true; }
+  _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; }
+  _LIBCPP_HIDE_FROM_ABI static constexpr bool is_exhaustive() noexcept { return true; }
+  _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(extents_type::rank() > 0)
+  {
+    _LIBCPP_ASSERT(__r < extents_type::rank(), "layout_left::mapping::stride(): invalid rank index");
+    index_type __s = 1;
+    for (rank_type __i = extents_type::rank() - 1; __i > __r; __i--)
+      __s *= __extents_.extent(__i);
+    return __s;
+  }
+
+  template <class _OtherExtents>
+    requires(_OtherExtents::rank() == extents_type::rank())
+  _LIBCPP_HIDE_FROM_ABI friend constexpr bool
+  operator==(const mapping& __lhs, const mapping<_OtherExtents>& __rhs) noexcept {
+    return __lhs.extents() == __rhs.extents();
+  }
+
+private:
+  extents_type __extents_{}; // exposition only
+};
+
+#endif // _LIBCPP_STD_VER >= 23
+
+_LIBCPP_END_NAMESPACE_STD
+
+_LIBCPP_POP_MACROS
+
+#endif // _LIBCPP___MDSPAN_LAYOUT_LEFT_H

libcxx/include/__mdspan/layout_right.h

public:
   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
       mapping(const mapping<_OtherExtents>& __other) noexcept
       : __extents_(__other.extents()) {
     _LIBCPP_ASSERT(
         __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
         "layout_right::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
 
-// FIXME: add when we add other layouts
-#  if 0
-    template<class _OtherExtents>
-      constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
-        mapping(const layout_left::mapping<_OtherExtents>&) noexcept {}
+  template <class _OtherExtents>
+    requires(is_constructible_v<extents_type, _OtherExtents> && _OtherExtents::rank() <= 1)
+  _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherExtents, extents_type>)
+      mapping(const layout_left::mapping<_OtherExtents>& __other) noexcept
+      : __extents_(__other.extents()) {
+    _LIBCPP_ASSERT(
+        __mdspan_detail::__is_representable_as<index_type>(__other.required_span_size()),
+        "layout_right::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+  }
 
+// FIXME: add when we add other layouts
+#  if 0
     template<class _OtherExtents>
       constexpr explicit(extents_type::rank() > 0)
         mapping(const layout_stride::mapping_<OtherExtents>&) noexcept;
 #  endif
 
   _LIBCPP_HIDE_FROM_ABI constexpr mapping& operator=(const mapping&) noexcept = default;
 
   // [mdspan.layout.right.obs], observers

libcxx/include/mdspan

namespace std {
   template<class IndexType, size_t... Extents>
     class extents;
 
   // [mdspan.extents.dextents], alias template dextents
   template<class IndexType, size_t Rank>
     using dextents = see below;
 
   // [mdspan.layout], layout mapping
-  struct layout_left; // not implemented yet
+  struct layout_left;
   struct layout_right;
   struct layout_stride; // not implemented yet
 
   // [mdspan.accessor.default], class template default_accessor
   template<class ElementType>
     class default_accessor; // not implemented yet
 
   // [mdspan.mdspan], class template mdspan
private:
     array<index_type, rank_dynamic()> dynamic-extents{};                // exposition only
   };
 
   template<class... Integrals>
     explicit extents(Integrals...)
       -> see below;
 }
 
+// layout_left synopsis
+
+namespace std {
+  template<class Extents>
+  class layout_left::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_left;
+
+    // [mdspan.layout.right.cons], constructors
+    constexpr mapping() noexcept = default;
+    constexpr mapping(const mapping&) noexcept = default;
+    constexpr mapping(const extents_type&) noexcept;
+    template<class OtherExtents>
+      constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+        mapping(const mapping<OtherExtents>&) noexcept;
+    template<class OtherExtents>
+      constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+        mapping(const layout_right::mapping<OtherExtents>&) noexcept;
+    template<class OtherExtents>
+      constexpr explicit(extents_type::rank() > 0)
+        mapping(const layout_stride::mapping<OtherExtents>&) noexcept;
+
+    constexpr mapping& operator=(const mapping&) noexcept = default;
+
+    // [mdspan.layout.right.obs], observers
+    constexpr const extents_type& extents() const noexcept { return extents_; }
+
+    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 true; }
+    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) const noexcept;
+
+    template<class OtherExtents>
+      friend constexpr bool operator==(const mapping&, const mapping<OtherExtents>&) noexcept;
+
+  private:
+    extents_type extents_{};    // exposition only
+  };
+}
+
 // layout_right synopsis
 
 namespace std {
   template<class Extents>
   class layout_right::mapping {
   public:
     using extents_type = Extents;
     using index_type = typename extents_type::index_type;

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

libcxx/include/module.modulemap.in

   module mdspan {
     header "mdspan"
     export array
     export span
     export *
 
     module __mdspan {
       module extents                         { private header "__mdspan/extents.h" }
+      module layout_left                     { private header "__mdspan/layout_left.h" }
       module layout_right                    { private header "__mdspan/layout_right.h" }
       module mdspan_fwd                      { private header "__fwd/mdspan.h" }
     }
   }
   module memory {
     header "memory"
     export *
 

libcxx/modules/std/mdspan.cppm

 export namespace std {
   // [mdspan.extents], class template extents
   using std::extents;
 
   // [mdspan.extents.dextents], alias template dextents
   using std::dextents;
 
   // [mdspan.layout], layout mapping
-  // using std::layout_left;
+  using std::layout_left;
   using std::layout_right;
   // using std::layout_stride;
 
 #if 0
   // [mdspan.accessor.default], class template default_­accessor
   using std::default_accessor;
 
   // [mdspan.mdspan], class template mdspan
   using std::mdspan;
 #endif
 } // namespace std

libcxx/test/std/containers/views/mdspan/layout_left/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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <mdspan>
+
+// template<class OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const mapping<OtherExtents>&) noexcept;
+
+// Constraints: is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type.
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  std::extents<int, D, D> arg_exts{100, 5};
+  std::layout_left::mapping<std::extents<int, D, D>> arg(arg_exts);
+
+  // working case
+  {
+    [[maybe_unused]] std::layout_left::mapping<std::extents<size_t, D, 5>> m(arg); // should work
+  }
+  // mismatch of static extent
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_left::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_left::mapping<std::extents<char, D>> m(
+                                 std::layout_left::mapping<std::extents<int, D>>(std::extents<int, D>(500))); }()),
+                               "extents ctor: arguments must be representable as index_type and nonnegative");
+  }
+  // required_span_size not representable, while individual extents are
+  {
+    // 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_left
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::layout_left::template mapping<std::extents<char, D, 5>> m(arg); }()),
+        "layout_left::mapping converting ctor: other.required_span_size() must be representable as index_type.");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/assert.ctor.extents.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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <mdspan>
+
+
+// constexpr mapping(const extents_type& e) noexcept;
+//
+// Preconditions: The size of the multidimensional index space e is representable as a value of type index_type ([basic.fundamental]).
+//
+// Effects: Direct-non-list-initializes extents_ with e.
+
+#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 is not, so we can't use it for layout_left
+    TEST_LIBCPP_ASSERT_FAILURE(
+        ([=] { std::layout_left::template mapping<std::extents<char, D, 5>> m(std::extents<char, D, 5>(100)); }()),
+        "layout_left::mapping extents ctor: product of extents must be representable as index_type.");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/assert.ctor.layout_right.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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <mdspan>
+
+// template<class OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const layout_right::mapping<OtherExtents>&) noexcept;
+
+// Constraints:
+//   - extents_type::rank() <= 1 is true, and 
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  std::extents<int, D> arg_exts{5};
+  std::layout_right::mapping<std::extents<int, D>> arg(arg_exts);
+
+  // working case
+  {
+    [[maybe_unused]] std::layout_left::mapping<std::extents<size_t, 5>> m(arg); // should work
+  }
+  // mismatch of static extent
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_left::mapping<std::extents<int, 3>> m(arg); }()),
+                               "extents construction: mismatch of provided arguments with static extents.");
+  }
+  // non-representability of extents itself
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_left::mapping<std::extents<char, D>> m(
+                                 std::layout_right::mapping<std::extents<int, D>>(std::extents<int, D>(500))); }()),
+                               "extents ctor: arguments must be representable as index_type and nonnegative");
+  }
+
+  // Can't trigger required_span_size() representability assertion, since for rank-1 the above check will trigger first,
+  // and this conversion constructor is constrained on rank <= 1.
+
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <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_left::template mapping<std::extents<unsigned char, 5>> m;
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_left::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_left::template mapping<std::extents<signed char, 5>> m;
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_left::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_left::template mapping<std::dextents<unsigned char, 1>> m(std::dextents<unsigned char, 1>(5));
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_left::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_left::template mapping<std::dextents<signed char, 1>> m(std::dextents<signed char, 1>(5));
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-130), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(1000), "layout_left::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_left::template mapping<std::dextents<int, 3>> m(std::dextents<int, 3>(5, 7, 9));
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, -1, -1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, 0, 0), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, -1, 0), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, 0, -1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 3, 3), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 7, 3), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 3, 9), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 7, 9), "layout_left::mapping: out of bounds indexing");
+  }
+  {
+    std::layout_left::template mapping<std::dextents<unsigned, 3>> m(std::dextents<int, 3>(5, 7, 9));
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, -1, -1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(-1, 0, 0), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, -1, 0), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(0, 0, -1), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 3, 3), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 7, 3), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(3, 3, 9), "layout_left::mapping: out of bounds indexing");
+    TEST_LIBCPP_ASSERT_FAILURE(m(5, 7, 9), "layout_left::mapping: out of bounds indexing");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <mdspan>
+
+// layout_left::mapping
+//
+// 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 "check_assertion.h"
+
+int main(int, char**) {
+  // value out of range
+  {
+    std::layout_left::template mapping<std::dextents<int, 3>> m{std::dextents<int, 3>{100, 100, 100}};
+
+    TEST_LIBCPP_ASSERT_FAILURE(m.stride(4), "invalid rank index");
+  }
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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 OtherExtents>
+//   friend constexpr bool operator==(const mapping& x, const mapping<OtherExtents>& y) noexcept;
+//                                      `
+// Constraints: extents_type::rank() == OtherExtents::rank() is true.
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class To, class From>
+constexpr void test_comparison(bool equal, To dest_exts, From src_exts) {
+  std::layout_left::mapping<To> dest(dest_exts);
+  std::layout_left::mapping<From> src(src_exts);
+  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, E2 e2)
+    -> decltype(std::layout_left::mapping<E1>(e1) == std::layout_left::mapping<E2>(e2)) {
+  return true;
+}
+
+template <class T1, class T2>
+constexpr void test_comparison_different_rank() {
+  constexpr size_t D = std::dynamic_extent;
+
+  // sanity check same rank
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), std::extents<T2, D>(5)));
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), std::extents<T2, D>(5)));
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), std::extents<T2, 5>()));
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), std::extents<T2, 5>()));
+
+  // not equality comparable when rank is not the same
+  static_assert(compare_layout_mappings(std::extents<T1>(), std::extents<T2, D>(1)).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1>(), std::extents<T2, 1>()).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D>(1), std::extents<T2>()).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 1>(), std::extents<T2>()).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D>(5), std::extents<T2, D, D>(5, 5)).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), std::extents<T2, 5, D>(5)).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5>(), std::extents<T2, 5, 1>()).does_not_match());
+
+  static_assert(compare_layout_mappings(std::extents<T1, D, D>(5, 5), std::extents<T2, D>(5)).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5, D>(5), std::extents<T2, D>(5)).does_not_match());
+  static_assert(compare_layout_mappings(std::extents<T1, 5, 5>(), std::extents<T2, 5>()).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>());
+
+  test_comparison(true, std::extents<T1, D>(5), std::extents<T2, D>(5));
+  test_comparison(true, std::extents<T1, 5>(), std::extents<T2, D>(5));
+  test_comparison(true, std::extents<T1, D>(5), std::extents<T2, 5>());
+  test_comparison(true, std::extents<T1, 5>(), std::extents< T2, 5>());
+  test_comparison(false, std::extents<T1, D>(5), std::extents<T2, D>(7));
+  test_comparison(false, std::extents<T1, 5>(), std::extents<T2, D>(7));
+  test_comparison(false, std::extents<T1, D>(5), std::extents<T2, 7>());
+  test_comparison(false, std::extents<T1, 5>(), std::extents<T2, 7>());
+
+  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));
+  test_comparison(true, std::extents<T1, D, 6, D, 8, D>(5, 7, 9), std::extents<T2, 5, D, D, 8, 9>(6, 7));
+  test_comparison(true, std::extents<T1, 5, 6, 7, 8, 9>(5, 6, 7, 8, 9), std::extents<T2, 5, 6, 7, 8, 9>());
+  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));
+  test_comparison(false, std::extents<T1, D, 6, D, 8, D>(5, 7, 9), std::extents<T2, 5, D, D, 3, 9>(6, 7));
+  test_comparison(false, std::extents<T1, 5, 6, 7, 8, 9>(5, 6, 7, 8, 9), std::extents<T2, 5, 6, 7, 3, 9>());
+}
+
+template <class T1, class T2>
+constexpr void test_comparison() {
+  test_comparison_same_rank<T1, T2>();
+  test_comparison_different_rank<T1, T2>();
+}
+
+constexpr bool test() {
+  test_comparison<int, int>();
+  test_comparison<int, size_t>();
+  test_comparison<size_t, int>();
+  test_comparison<size_t, long>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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 = default;
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_construction() {
+  using M = std::layout_left::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);
+}
+
+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, 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_left/ctor.extents.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 mapping(const extents_type&) noexcept;
+//
+// Preconditions: The size of the multidimensional index space e is representable
+//                as a value of type index_type ([basic.fundamental]).
+//
+// Effects: Direct-non-list-initializes extents_ with e.
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_construction(E e) {
+  using M = std::layout_left::mapping<E>;
+  ASSERT_NOEXCEPT(M{e});
+  M m(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);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_construction(std::extents<int>());
+  test_construction(std::extents<unsigned, D>(7));
+  test_construction(std::extents<unsigned, 7>());
+  test_construction(std::extents<unsigned, 7, 8>());
+  test_construction(std::extents<int64_t, D, 8, D, D>(7, 9, 10));
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/ctor.layout_right.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 OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const layout_right::mapping<OtherExtents>&) noexcept;
+
+// Constraints:
+//   - extents_type::rank() <= 1 is true, and 
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type
+
+#include <mdspan>
+#include <type_traits>
+#include <cassert>
+#include <limits>
+
+#include "test_macros.h"
+
+template <class To, class From>
+constexpr void test_implicit_conversion(To dest, From src) {
+  assert(dest.extents() == src.extents());
+}
+
+template <bool implicit, class ToE, class FromE>
+constexpr void test_conversion(FromE src_exts) {
+  using To   = std::layout_left::mapping<ToE>;
+  using From = std::layout_right::mapping<FromE>;
+  From src(src_exts);
+
+  ASSERT_NOEXCEPT(To(src));
+  To dest(src);
+
+  assert(dest.extents() == src.extents());
+  if constexpr (implicit) {
+    dest = src;
+    assert(dest.extents() == src.extents());
+    test_implicit_conversion<To, From>(src, src);
+  }
+}
+
+template <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());
+
+  // clang-format off
+  test_conversion<idx_convertible && true,  std::extents<T1>>(std::extents<T2>());
+  test_conversion<idx_convertible && true,  std::extents<T1, D>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && false, std::extents<T1, 5>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && true,  std::extents<T1, 5>>(std::extents<T2, 5>());
+  // clang-format on
+}
+
+template <class IdxT, size_t... Extents>
+using lr_mapping_t = typename std::layout_right::template mapping<std::extents<IdxT, Extents...>>;
+template <class IdxT, size_t... Extents>
+using ll_mapping_t = typename std::layout_left::template mapping<std::extents<IdxT, Extents...>>;
+
+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<ll_mapping_t<int, D>, lr_mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<lr_mapping_t<int, 5>, ll_mapping_t<int, D>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<ll_mapping_t<int, 5>, lr_mapping_t<int, D>>);
+  static_assert(!std::is_convertible_v<lr_mapping_t<int, D>, ll_mapping_t<int, 5>>);
+
+  // Sanity check that smaller index_type to larger index_type conversion works
+  static_assert(std::is_constructible_v<ll_mapping_t<size_t, 5>, lr_mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<lr_mapping_t<int, 5>, ll_mapping_t<size_t, 5>>);
+
+  // Check that larger index_type to smaller index_type conversion works explicitly only
+  static_assert(std::is_constructible_v<ll_mapping_t<int, 5>, lr_mapping_t<size_t, 5>>);
+  static_assert(!std::is_convertible_v<lr_mapping_t<size_t, 5>, ll_mapping_t<int, 5>>);
+}
+
+constexpr void test_rank_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D>, lr_mapping_t<int>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int>, lr_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D>, lr_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D, D, D>, lr_mapping_t<int, D, D>>);
+}
+
+constexpr void test_static_extent_mismatch() {
+  static_assert(!std::is_constructible_v<ll_mapping_t<int, 5>, lr_mapping_t<int, 4>>);
+}
+
+constexpr void test_rank_greater_one() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<ll_mapping_t<int, D, D>, lr_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<ll_mapping_t<int, 1, 1>, lr_mapping_t<int, 1, 1>>);
+  static_assert(!std::is_constructible_v<ll_mapping_t<int, D, D, D>, lr_mapping_t<int, D, D, D>>);
+}
+
+constexpr bool test() {
+  test_conversion<int, int>();
+  test_conversion<int, size_t>();
+  test_conversion<size_t, int>();
+  test_conversion<size_t, long>();
+  test_no_implicit_conversion();
+  test_rank_mismatch();
+  test_static_extent_mismatch();
+  test_rank_greater_one();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/ctor.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 OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const mapping<OtherExtents>&) noexcept;
+
+// Constraints: is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type
+
+#include <mdspan>
+#include <type_traits>
+#include <cassert>
+#include <limits>
+
+#include "test_macros.h"
+
+template <class To, class From>
+constexpr void test_implicit_conversion(To dest, From src) {
+  assert(dest == src);
+}
+
+template <bool implicit, class ToE, class FromE>
+constexpr void test_conversion(FromE src_exts) {
+  using To   = std::layout_left::mapping<ToE>;
+  using From = std::layout_left::mapping<FromE>;
+  From src(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);
+  }
+}
+
+template <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());
+
+  // clang-format off
+  test_conversion<idx_convertible && true,  std::extents<T1>>(std::extents<T2>());
+  test_conversion<idx_convertible && true,  std::extents<T1, D>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && false, std::extents<T1, 5>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && true,  std::extents<T1, 5>>(std::extents<T2, 5>());
+  test_conversion<idx_convertible && false, std::extents<T1, 5, D>>(std::extents<T2, D, D>(5, 5));
+  test_conversion<idx_convertible && true,  std::extents<T1, D, D>>(std::extents<T2, D, D>(5, 5));
+  test_conversion<idx_convertible && true,  std::extents<T1, D, D>>(std::extents<T2, D, 7>(5));
+  test_conversion<idx_convertible && true,  std::extents<T1, 5, 7>>(std::extents<T2, 5, 7>());
+  test_conversion<idx_convertible && false, std::extents<T1, 5, D, 8, D, D>>(std::extents<T2, D, D, 8, 9, 1>(5, 7));
+  test_conversion<idx_convertible && true,  std::extents<T1, D, D, D, D, D>>(
+                                            std::extents<T2, D, D, D, D, D>(5, 7, 8, 9, 1));
+  test_conversion<idx_convertible && true,  std::extents<T1, D, D, 8, 9, D>>(std::extents<T2, D, 7, 8, 9, 1>(5));
+  test_conversion<idx_convertible && true,  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 mapping_t = typename std::layout_left::template mapping<std::extents<IdxT, Extents...>>;
+
+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<mapping_t<int, D>, mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<mapping_t<int, 5>, mapping_t<int, D>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<mapping_t<int, 5>, mapping_t<int, D>>);
+  static_assert(!std::is_convertible_v<mapping_t<int, D>, mapping_t<int, 5>>);
+
+  // Sanity check that one static to dynamic conversion works
+  static_assert(std::is_constructible_v<mapping_t<int, D, 7>, mapping_t<int, 5, 7>>);
+  static_assert(std::is_convertible_v<mapping_t<int, 5, 7>, mapping_t<int, D, 7>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<mapping_t<int, 5, 7>, mapping_t<int, D, 7>>);
+  static_assert(!std::is_convertible_v<mapping_t<int, D, 7>, mapping_t<int, 5, 7>>);
+
+  // Sanity check that smaller index_type to larger index_type conversion works
+  static_assert(std::is_constructible_v<mapping_t<size_t, 5>, mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<mapping_t<int, 5>, mapping_t<size_t, 5>>);
+
+  // Check that larger index_type to smaller index_type conversion works explicitly only
+  static_assert(std::is_constructible_v<mapping_t<int, 5>, mapping_t<size_t, 5>>);
+  static_assert(!std::is_convertible_v<mapping_t<size_t, 5>, mapping_t<int, 5>>);
+}
+
+constexpr void test_rank_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<mapping_t<int, D>, mapping_t<int>>);
+  static_assert(!std::is_constructible_v<mapping_t<int>, mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<mapping_t<int, D>, mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<mapping_t<int, D, D, D>, mapping_t<int, D, D>>);
+}
+
+constexpr void test_static_extent_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<mapping_t<int, D, 5>, mapping_t<int, D, 4>>);
+  static_assert(!std::is_constructible_v<mapping_t<int, 5>, mapping_t<int, 4>>);
+  static_assert(!std::is_constructible_v<mapping_t<int, 5, D>, mapping_t<int, 4, D>>);
+}
+
+constexpr bool test() {
+  test_conversion<int, int>();
+  test_conversion<int, size_t>();
+  test_conversion<size_t, int>();
+  test_conversion<size_t, long>();
+  test_no_implicit_conversion();
+  test_rank_mismatch();
+  test_static_extent_mismatch();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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
+//
+//===----------------------------------------------------------------------===//
+// REQUIRES: has-unix-headers
+// UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+
+// <mdspan>
+
+// template<class Extents>
+// class layout_left::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_left::mapping template argument must be a specialization of extents}}
+  [[maybe_unused]] std::layout_left::mapping<void> mapping;
+}
+
+void representable() {
+  // expected-error-re@*:* {{{{(static_assert|static assertion)}} failed {{.*}}layout_left::mapping product of static extents must be representable as index_type.}}
+  [[maybe_unused]] std::layout_left::mapping<std::extents<char, 20, 20>> mapping;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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 T, class... Args>
+constexpr void iterate_left(M m, T& count, 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...));
+    assert(count == m(args...));
+    count++;
+  } else {
+    for (typename M::index_type i = 0; i < m.extents().extent(r); i++) {
+      iterate_left(m, count, i, args...);
+    }
+  }
+}
+
+template <class E, class... Args>
+constexpr void test_iteration(Args... args) {
+  using M = std::layout_left::mapping<E>;
+  M m(E(args...));
+
+  typename E::index_type count = 0;
+  iterate_left(m, count);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_iteration<std::extents<int>>();
+  test_iteration<std::extents<unsigned, D>>(1);
+  test_iteration<std::extents<unsigned, D>>(7);
+  test_iteration<std::extents<unsigned, 7>>();
+  test_iteration<std::extents<unsigned, 7, 8>>();
+  test_iteration<std::extents<int64_t, D, 8, D, D>>(7, 9, 10);
+  test_iteration<std::extents<int64_t, D, 8, 1, D>>(7, 10);
+  test_iteration<std::extents<char, D, D, D, D>>(1, 1, 1, 1);
+
+  // Check operator constraint for number of arguments
+  static_assert(check_operator_constraints(std::layout_left::mapping<std::extents<int, D>>(std::extents<int, D>(1)), 0));
+  static_assert(!check_operator_constraints(std::layout_left::mapping<std::extents<int, D>>(std::extents<int, D>(1)), 0, 0));
+
+  // Check operator constraint for convertibility of arguments to index_type
+  static_assert(check_operator_constraints(std::layout_left::mapping<std::extents<int, D>>(std::extents<int, D>(1)), IntType(0)));
+  static_assert(!check_operator_constraints(std::layout_left::mapping<std::extents<unsigned, D>>(std::extents<unsigned, D>(1)), IntType(0)));
+
+  // Check operator constraint for no-throw-constructibility of index_type from arguments
+  static_assert(!check_operator_constraints(std::layout_left::mapping<std::extents<unsigned char, D>>(std::extents<unsigned char, D>(1)), IntType(0)));
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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_left::mapping {
+//
+//     ...
+//     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 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; }
+//     ...
+//   };
+// }
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts>
+#include <cassert>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_layout_mapping_left() {
+  using M = std::layout_left::template mapping<E>;
+  assert(M::is_unique() == true);
+  assert(M::is_exhaustive() == true);
+  assert(M::is_strided() == true);
+  assert(M::is_always_unique() == true);
+  assert(M::is_always_exhaustive() == true);
+  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());
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_layout_mapping_left<std::extents<int>>();
+  test_layout_mapping_left<std::extents<char, 4, 5>>();
+  test_layout_mapping_left<std::extents<unsigned, D, 4>>();
+  test_layout_mapping_left<std::extents<size_t, D, D, D, D>>();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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>
+
+// constexpr mapping(const extents_type&) noexcept;
+//
+// Preconditions: The size of the multidimensional index space e is representable
+//                as a value of type index_type ([basic.fundamental]).
+//
+// Effects: Direct-non-list-initializes extents_ with e.
+
+#include <mdspan>
+#include <cassert>
+#include <cstdint>
+
+#include "test_macros.h"
+
+template <class E>
+constexpr void test_construction(E e, typename E::index_type expected_size) {
+  using M = std::layout_left::mapping<E>;
+  const M m(e);
+
+  // check required_span_size()
+  ASSERT_NOEXCEPT(m.required_span_size());
+  assert(m.required_span_size() == expected_size);
+}
+
+constexpr bool test() {
+  constexpr size_t D = std::dynamic_extent;
+  test_construction(std::extents<int>(), 1);
+  test_construction(std::extents<unsigned, D>(0), 0);
+  test_construction(std::extents<unsigned, D>(1), 1);
+  test_construction(std::extents<unsigned, D>(7), 7);
+  test_construction(std::extents<unsigned, 7>(), 7);
+  test_construction(std::extents<unsigned, 7, 8>(), 56);
+  test_construction(std::extents<int64_t, D, 8, D, D>(7, 9, 10), 5040);
+  test_construction(std::extents<int64_t, 1, 8, D, D>(9, 10), 720);
+  test_construction(std::extents<int64_t, 1, 0, D, D>(9, 10), 0);
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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_left);
+  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>()(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_left() {
+  test_layout_mapping_requirements<std::layout_left, E>();
+}
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  test_layout_mapping_left<std::extents<int>>();
+  test_layout_mapping_left<std::extents<char, 4, 5>>();
+  test_layout_mapping_left<std::extents<unsigned, D, 4>>();
+  test_layout_mapping_left<std::extents<size_t, D, D, D, D>>();
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_left/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_left::mapping<E>;
+  M m(E(args...));
+
+  ASSERT_NOEXCEPT(m.stride(0));
+  for (size_t r = 0; r < E::rank(); r++)
+    assert(strides[r] == m.stride(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/layout_right/assert.conversion.pass.cpp

int main(int, char**) {
   {
     [[maybe_unused]] std::layout_right::mapping<std::extents<size_t, D, 5>> m(arg); // should work
   }
   // mismatch of static extent
   {
     TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_right::mapping<std::extents<int, D, 3>> m(arg); }()),
                                "extents construction: mismatch of provided arguments with static extents.");
   }
-  // value out of range
+  // non-representability of extents itself
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_right::mapping<std::extents<char, D>> m(
+                                 std::layout_right::mapping<std::extents<int, D>>(std::extents<int, D>(500))); }()),
+                               "extents ctor: arguments must be representable as index_type and nonnegative");
+  }
+  // required_span_size not representable, while individual extents are
   {
     // 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_right
     TEST_LIBCPP_ASSERT_FAILURE(
         ([=] { std::layout_right::template mapping<std::extents<char, D, 5>> m(arg); }()),
         "layout_right::mapping converting ctor: other.required_span_size() must be representable as index_type.");
   }
   return 0;
 }

libcxx/test/std/containers/views/mdspan/layout_right/assert.ctor.layout_left.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
+// XFAIL: availability-verbose_abort-missing
+// ADDITIONAL_COMPILE_FLAGS: -D_LIBCPP_ENABLE_ASSERTIONS=1
+
+// <mdspan>
+
+// template<class OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const layout_left::mapping<OtherExtents>&) noexcept;
+
+// Constraints:
+//   - extents_type::rank() <= 1 is true, and 
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type
+
+#include <mdspan>
+#include <cassert>
+
+#include "check_assertion.h"
+
+int main(int, char**) {
+  constexpr size_t D = std::dynamic_extent;
+  std::extents<int, D> arg_exts{5};
+  std::layout_left::mapping<std::extents<int, D>> arg(arg_exts);
+
+  // working case
+  {
+    [[maybe_unused]] std::layout_right::mapping<std::extents<size_t, 5>> m(arg); // should work
+  }
+  // mismatch of static extent
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_right::mapping<std::extents<int, 3>> m(arg); }()),
+                               "extents construction: mismatch of provided arguments with static extents.");
+  }
+  // non-representability of extents itself
+  {
+    TEST_LIBCPP_ASSERT_FAILURE(([=] { std::layout_right::mapping<std::extents<char, D>> m(
+                                 std::layout_left::mapping<std::extents<int, D>>(std::extents<int, D>(500))); }()),
+                               "extents ctor: arguments must be representable as index_type and nonnegative");
+  }
+
+  // Can't trigger required_span_size() representability assertion, since for rank-1 the above check will trigger first,
+  // and this conversion constructor is constrained on rank <= 1.
+  return 0;
+}

libcxx/test/std/containers/views/mdspan/layout_right/ctor.layout_left.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 OtherExtents>
+//   constexpr explicit(!is_convertible_v<OtherExtents, extents_type>)
+//     mapping(const layout_left::mapping<OtherExtents>&) noexcept;
+
+// Constraints:
+//   - extents_type::rank() <= 1 is true, and 
+//   - is_constructible_v<extents_type, OtherExtents> is true.
+//
+// Preconditions: other.required_span_size() is representable as a value of type index_type
+
+#include <mdspan>
+#include <type_traits>
+#include <concepts> // what do we use from there?
+#include <cassert>
+#include <limits>
+
+#include "test_macros.h"
+
+template <class To, class From>
+constexpr void test_implicit_conversion(To dest, From src) {
+  assert(dest.extents() == src.extents());
+}
+
+template <bool implicit, class ToE, class FromE>
+constexpr void test_conversion(FromE src_exts) {
+  using To   = std::layout_right::mapping<ToE>;
+  using From = std::layout_left::mapping<FromE>;
+  From src(src_exts);
+
+  ASSERT_NOEXCEPT(To(src));
+  To dest(src);
+
+  assert(dest.extents() == src.extents());
+  if constexpr (implicit) {
+    dest = src;
+    assert(dest.extents() == src.extents());
+    test_implicit_conversion<To, From>(src, src);
+  }
+}
+
+template <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());
+
+  // clang-format off
+  test_conversion<idx_convertible && true,  std::extents<T1>>(std::extents<T2>());
+  test_conversion<idx_convertible && true,  std::extents<T1, D>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && false, std::extents<T1, 5>>(std::extents<T2, D>(5));
+  test_conversion<idx_convertible && true,  std::extents<T1, 5>>(std::extents<T2, 5>());
+  // clang-format on
+}
+
+template <class IdxT, size_t... Extents>
+using lr_mapping_t = typename std::layout_right::template mapping<std::extents<IdxT, Extents...>>;
+template <class IdxT, size_t... Extents>
+using ll_mapping_t = typename std::layout_left::template mapping<std::extents<IdxT, Extents...>>;
+
+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<lr_mapping_t<int, D>, ll_mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<ll_mapping_t<int, 5>, lr_mapping_t<int, D>>);
+
+  // Check that dynamic to static conversion only works explicitly
+  static_assert(std::is_constructible_v<lr_mapping_t<int, 5>, ll_mapping_t<int, D>>);
+  static_assert(!std::is_convertible_v<ll_mapping_t<int, D>, lr_mapping_t<int, 5>>);
+
+  // Sanity check that smaller index_type to larger index_type conversion works
+  static_assert(std::is_constructible_v<lr_mapping_t<size_t, 5>, ll_mapping_t<int, 5>>);
+  static_assert(std::is_convertible_v<ll_mapping_t<int, 5>, lr_mapping_t<size_t, 5>>);
+
+  // Check that larger index_type to smaller index_type conversion works explicitly only
+  static_assert(std::is_constructible_v<lr_mapping_t<int, 5>, ll_mapping_t<size_t, 5>>);
+  static_assert(!std::is_convertible_v<ll_mapping_t<size_t, 5>, lr_mapping_t<int, 5>>);
+}
+
+constexpr void test_rank_mismatch() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D>, ll_mapping_t<int>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int>, ll_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D>, ll_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D, D, D>, ll_mapping_t<int, D, D>>);
+}
+
+constexpr void test_static_extent_mismatch() {
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, 5>, ll_mapping_t<int, 4>>);
+}
+
+constexpr void test_rank_greater_one() {
+  constexpr size_t D = std::dynamic_extent;
+
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D, D>, ll_mapping_t<int, D, D>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, 1, 1>, ll_mapping_t<int, 1, 1>>);
+  static_assert(!std::is_constructible_v<lr_mapping_t<int, D, D, D>, ll_mapping_t<int, D, D, D>>);
+}
+
+constexpr bool test() {
+  test_conversion<int, int>();
+  test_conversion<int, size_t>();
+  test_conversion<size_t, int>();
+  test_conversion<size_t, long>();
+  test_no_implicit_conversion();
+  test_rank_mismatch();
+  test_static_extent_mismatch();
+  test_rank_greater_one();
+  return true;
+}
+
+int main(int, char**) {
+  test();
+  static_assert(test());
+  return 0;
+}