diff --git a/stl/inc/mdspan b/stl/inc/mdspan
index 5bc6c74f0f1..adbe8e48913 100644
--- a/stl/inc/mdspan
+++ b/stl/inc/mdspan
@@ -13,6 +13,7 @@ _EMIT_STL_WARNING(STL4038, "The contents of <mdspan> are available only with C++
 #else // ^^^ not supported / supported language mode vvv
 #include <array>
 #include <span>
+#include <type_traits>
 
 #pragma pack(push, _CRT_PACKING)
 #pragma warning(push, _STL_WARNING_LEVEL)
@@ -201,7 +202,7 @@ public:
         }
 
         for (size_t _Dim = 0; _Dim < sizeof...(_Extents); ++_Dim) {
-            if (_Left.extent(_Dim) != _Right.extent(_Dim)) {
+            if (_STD cmp_not_equal(_Left.extent(_Dim), _Right.extent(_Dim))) {
                 return false;
             }
         }
diff --git a/tests/std/test.lst b/tests/std/test.lst
index a70c5617cb2..b6b81ee1106 100644
--- a/tests/std/test.lst
+++ b/tests/std/test.lst
@@ -228,6 +228,8 @@ tests\LWG3480_directory_iterator_range
 tests\LWG3545_pointer_traits_sfinae
 tests\LWG3610_iota_view_size_and_integer_class
 tests\P0009R18_mdspan
+tests\P0009R18_mdspan_default_accessor
+tests\P0009R18_mdspan_extents
 tests\P0019R8_atomic_ref
 tests\P0024R2_parallel_algorithms_adjacent_difference
 tests\P0024R2_parallel_algorithms_adjacent_find
diff --git a/tests/std/tests/P0009R18_mdspan_default_accessor/env.lst b/tests/std/tests/P0009R18_mdspan_default_accessor/env.lst
new file mode 100644
index 00000000000..18e2d7c71ec
--- /dev/null
+++ b/tests/std/tests/P0009R18_mdspan_default_accessor/env.lst
@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_latest_matrix.lst
diff --git a/tests/std/tests/P0009R18_mdspan_default_accessor/test.cpp b/tests/std/tests/P0009R18_mdspan_default_accessor/test.cpp
new file mode 100644
index 00000000000..21379295e76
--- /dev/null
+++ b/tests/std/tests/P0009R18_mdspan_default_accessor/test.cpp
@@ -0,0 +1,66 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <cstdint>
+#include <mdspan>
+#include <string>
+#include <type_traits>
+
+using namespace std;
+
+template <class ElementType>
+constexpr void test_one(array<ElementType, 3> elems) {
+    using Accessor = default_accessor<ElementType>;
+
+    // Check modeled concepts
+    static_assert(is_nothrow_move_constructible_v<Accessor>);
+    static_assert(is_nothrow_move_assignable_v<Accessor>);
+    static_assert(is_nothrow_swappable_v<Accessor>);
+    static_assert(is_trivially_copyable_v<Accessor>);
+    static_assert(semiregular<Accessor>);
+
+    // Check nested types
+    static_assert(same_as<typename Accessor::offset_policy, Accessor>);
+    static_assert(same_as<typename Accessor::element_type, ElementType>);
+    static_assert(same_as<typename Accessor::reference, ElementType&>);
+    static_assert(same_as<typename Accessor::data_handle_type, ElementType*>);
+
+    // Check default constructor
+    Accessor accessor;
+    static_assert(is_nothrow_default_constructible_v<Accessor>);
+
+    { // Check converting constructor from other accessor
+        [[maybe_unused]] default_accessor<const ElementType> const_accessor = accessor;
+        static_assert(is_nothrow_constructible_v<default_accessor<const ElementType>, Accessor>);
+        static_assert(!is_constructible_v<Accessor, default_accessor<const ElementType>>);
+    }
+
+    { // Check 'access' member function
+        same_as<ElementType&> decltype(auto) accessed_elem = accessor.access(elems.data(), 1);
+        assert(accessed_elem == elems[1]);
+        static_assert(noexcept(accessor.access(elems.data(), 0)));
+    }
+
+    { // Check 'offset' member function
+        same_as<ElementType*> auto ptr = accessor.offset(elems.data(), 1);
+        assert(ptr == elems.data() + 1);
+        static_assert(noexcept(accessor.offset(elems.data(), 0)));
+    }
+}
+
+constexpr bool test() {
+    test_one<char>({'a', 'b', 'c'});
+    test_one<long>({1, 2, 3});
+    test_one<double>({1.1, 2.2, 3.3});
+    test_one<wstring>({L"1", L"2", L"3"});
+    test_one<intmax_t>({3, 2, 1});
+    return true;
+}
+
+int main() {
+    static_assert(test());
+    test();
+}
diff --git a/tests/std/tests/P0009R18_mdspan_extents/env.lst b/tests/std/tests/P0009R18_mdspan_extents/env.lst
new file mode 100644
index 00000000000..18e2d7c71ec
--- /dev/null
+++ b/tests/std/tests/P0009R18_mdspan_extents/env.lst
@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\concepts_latest_matrix.lst
diff --git a/tests/std/tests/P0009R18_mdspan_extents/test.cpp b/tests/std/tests/P0009R18_mdspan_extents/test.cpp
new file mode 100644
index 00000000000..b73415b842d
--- /dev/null
+++ b/tests/std/tests/P0009R18_mdspan_extents/test.cpp
@@ -0,0 +1,295 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <array>
+#include <cassert>
+#include <concepts>
+#include <mdspan>
+#include <span>
+#include <type_traits>
+#include <utility>
+
+using namespace std;
+
+enum class IsNothrow : bool { no, yes };
+
+template <class Int, IsNothrow Nothrow = IsNothrow::yes>
+struct ConvertibleToInt {
+    constexpr operator Int() const noexcept(to_underlying(Nothrow)) {
+        return Int{1};
+    }
+};
+
+struct NonConvertibleToAnything {};
+
+template <class T>
+constexpr void check_implicit_conversion(T); // not defined
+
+// clang-format off
+template <class T, class... Args>
+concept NotImplicitlyConstructibleFrom =
+    constructible_from<T, Args...>
+    && !requires(Args&&... args) { check_implicit_conversion<T>({forward<Args>(args)...}); };
+// clang-format on
+
+template <class IndexType, size_t... Extents, size_t... Indices>
+constexpr void do_check_members(index_sequence<Indices...>) {
+    using Ext = extents<IndexType, Extents...>;
+
+    // Each specialization of extents models regular and is trivially copyable
+    static_assert(regular<Ext>);
+    static_assert(is_trivially_copyable_v<Ext>);
+
+    // Check member types
+    static_assert(same_as<typename Ext::index_type, IndexType>);
+    static_assert(same_as<typename Ext::size_type, make_unsigned_t<IndexType>>);
+    static_assert(same_as<typename Ext::rank_type, size_t>);
+
+    // Check static observers
+    static_assert(Ext::rank() == sizeof...(Extents));
+    static_assert(Ext::rank_dynamic() == ((Extents == dynamic_extent) + ... + 0));
+    static_assert(((Ext::static_extent(Indices) == Extents) && ...));
+
+    // Check noexceptness of static observers
+    static_assert(noexcept(Ext::rank()));
+    static_assert(noexcept(Ext::rank_dynamic()));
+    static_assert(noexcept(Ext::static_extent(0)));
+
+    // Check default constructor
+    Ext ext;
+    static_assert(is_nothrow_default_constructible_v<Ext>);
+
+    // Check 'extent' observer
+    assert((((ext.extent(Indices) == Extents && Extents != dynamic_extent) || ext.extent(Indices) == 0) && ...));
+
+    using OtherIndexType = conditional_t<is_signed_v<IndexType>, long long, unsigned long long>;
+    using Ext2           = extents<OtherIndexType, Extents...>;
+
+    { // Check construction from other extents
+        Ext2 ext2{ext};
+        assert(((ext.extent(Indices) == ext2.extent(Indices)) && ...));
+        assert(ext == ext2);
+        static_assert(is_nothrow_constructible_v<Ext2, Ext>);
+        // Other tests are defined in 'check_construction_from_other_extents' function
+    }
+
+    { // Check construction from extents pack
+        Ext2 ext2{ext.extent(Indices)...};
+        assert(((ext.extent(Indices) == ext2.extent(Indices)) && ...));
+        assert(ext == ext2);
+        static_assert(is_nothrow_constructible_v<Ext2, Ext>);
+        // Other tests are defined in 'check_construction_from_extents_pack' function
+    }
+
+    { // Check construction from array and span
+        auto arr = to_array<IndexType>({ext.extent(Indices)...});
+        Ext2 ext2a{arr};
+        assert(((ext.extent(Indices) == ext2a.extent(Indices)) && ...));
+        assert(ext == ext2a);
+        static_assert(is_nothrow_constructible_v<Ext2, decltype(arr)>);
+
+        span s{arr};
+        Ext2 ext2b{s};
+        assert(((ext.extent(Indices) == ext2b.extent(Indices)) && ...));
+        assert(ext == ext2b);
+        static_assert(is_nothrow_constructible_v<Ext2, decltype(s)>);
+        // Other tests are defined in 'check_construction_from_array_and_span' function
+    }
+}
+
+template <class IndexType, size_t... Extents>
+constexpr void check_members() {
+    do_check_members<IndexType, Extents...>(make_index_sequence<sizeof...(Extents)>{});
+}
+
+constexpr void check_construction_from_other_extents() {
+    { // Check construction from too big or too small other extents
+        using Ext = extents<int, 3, 3>;
+        static_assert(!is_constructible_v<Ext, int>);
+        static_assert(!is_constructible_v<Ext, extents<int, 3, 3, 3>>);
+    }
+
+    { // Check construction with different values
+        static_assert(is_nothrow_constructible_v<extents<int, 3, 3>, extents<int, 3, 3>>);
+        static_assert(is_nothrow_constructible_v<extents<int, 3, 3>, extents<int, 3, dynamic_extent>>);
+        static_assert(is_nothrow_constructible_v<extents<int, 3, dynamic_extent>, extents<int, 3, 3>>);
+        static_assert(is_nothrow_constructible_v<extents<int, 3, dynamic_extent>, extents<int, 3, dynamic_extent>>);
+        static_assert(!is_constructible_v<extents<int, 3, 3>, extents<int, 3, 2>>);
+    }
+
+    { // Check postconditions
+        extents<int, dynamic_extent, dynamic_extent> ext{4, 4};
+        extents<int, 4, 4> ext2{ext};
+        assert(ext == ext2);
+        assert(ext2.extent(0) == 4);
+        assert(ext2.extent(1) == 4);
+
+        extents<long, 4, dynamic_extent> ext3{ext};
+        assert(ext == ext3);
+        assert(ext3.extent(0) == 4);
+        assert(ext3.extent(1) == 4);
+    }
+
+    { // Check implicit conversions
+        static_assert(!NotImplicitlyConstructibleFrom<extents<int, 3>, extents<int, 3>>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3>, extents<long long, 3>>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3, 3>, extents<int, dynamic_extent, 3>>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3, 3>, extents<int, dynamic_extent, dynamic_extent>>);
+    }
+}
+
+constexpr void check_construction_from_extents_pack() {
+    { // Check construction from various types
+        using Ext = extents<int, 3, 3, dynamic_extent, dynamic_extent>;
+        static_assert(is_nothrow_constructible_v<Ext, int, int>);
+        static_assert(!is_constructible_v<Ext, int, int, int>);
+        static_assert(is_nothrow_constructible_v<Ext, int, int, int, int>);
+    }
+
+    { // Check construction from types (not) convertible to index_type
+        using Ext = extents<long, 2, dynamic_extent>;
+        static_assert(is_nothrow_constructible_v<Ext, ConvertibleToInt<int>>);
+        static_assert(!is_constructible_v<Ext, NonConvertibleToAnything>);
+        static_assert(is_nothrow_constructible_v<Ext, int, ConvertibleToInt<int>>);
+        static_assert(!is_constructible_v<Ext, int, NonConvertibleToAnything>);
+    }
+
+    { // Check construction from types that may throw during conversion to index_type
+        using Ext = extents<long long, 4, dynamic_extent>;
+        static_assert(!is_constructible_v<Ext, ConvertibleToInt<int, IsNothrow::no>>);
+        static_assert(!is_constructible_v<Ext, int, ConvertibleToInt<int, IsNothrow::no>>);
+    }
+
+#if 0 // FIXME Bug in array/span constructor?
+    { // Check postconditions [FIXME]
+        using Ext         = extents<int, dynamic_extent, dynamic_extent, 4>;
+        array<int, 3> arr = {4, 4, 4};
+        Ext ext{arr};
+        Ext ext2{4, 4, 4};
+        assert(ext == ext2);
+    }
+#endif // Bug?
+
+    { // Check implicit conversions
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3>, unsigned long long>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3, 3>, long, long>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<int, 3, 3, 3>, char, signed char, unsigned char>);
+    }
+}
+
+constexpr void check_construction_from_array_and_span() {
+    { // Check construction from arrays/spans with elements (not) convertible to index_type
+        using Ext = extents<short, 4, dynamic_extent>;
+
+        array<int, 2> arr1 = {4, 5};
+        Ext ext1a{arr1};
+        span s1{arr1};
+        Ext ext1b{s1};
+        assert(ext1a == ext1b);
+        static_assert(is_nothrow_constructible_v<Ext, decltype(arr1)>);
+        static_assert(is_nothrow_constructible_v<Ext, decltype(s1)>);
+
+        array<ConvertibleToInt<int>, 2> arr2;
+        Ext ext2a{arr2};
+        span s2{arr2};
+        Ext ext2b{s2};
+        assert(ext2a == ext2b);
+        static_assert(is_nothrow_constructible_v<Ext, decltype(arr2)>);
+        static_assert(is_nothrow_constructible_v<Ext, decltype(s2)>);
+
+        static_assert(!is_constructible_v<Ext, array<NonConvertibleToAnything, 2>>);
+        static_assert(!is_constructible_v<Ext, span<NonConvertibleToAnything, 2>>);
+    }
+
+    { // Check construction from arrays/spans with elements that may throw during conversion to index_type
+        using Ext = extents<long, dynamic_extent, dynamic_extent>;
+        static_assert(!is_constructible_v<Ext, array<ConvertibleToInt<int, IsNothrow::no>, 2>>);
+        static_assert(!is_constructible_v<Ext, span<ConvertibleToInt<int, IsNothrow::no>, 2>>);
+    }
+
+    { // Check construction from arrays/spans with invalid size
+        using Ext = extents<short, 5, 5>;
+        static_assert(!is_constructible_v<Ext, array<int, 1>>);
+        static_assert(!is_constructible_v<Ext, array<int, 3>>);
+        static_assert(!is_constructible_v<Ext, span<int, 1>>);
+        static_assert(!is_constructible_v<Ext, span<int, 3>>);
+        static_assert(!is_constructible_v<Ext, span<int>>);
+        static_assert(!is_constructible_v<Ext, span<int>>);
+    }
+
+    { // Check implicit conversions
+        static_assert(!NotImplicitlyConstructibleFrom<extents<signed char, 3, dynamic_extent>, array<int, 1>>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<signed char, 3, dynamic_extent>, array<int, 2>>);
+        static_assert(!NotImplicitlyConstructibleFrom<extents<signed char, 3, dynamic_extent>, span<int, 1>>);
+        static_assert(NotImplicitlyConstructibleFrom<extents<signed char, 3, dynamic_extent>, span<int, 2>>);
+    }
+}
+
+constexpr void check_equality_operator() {
+    { // All extents are static
+        extents<int, 3, 3> e1;
+        extents<long, 2, 3> e2;
+        extents<size_t, 3, 3> e3;
+        assert(e1 != e2);
+        assert(e2 != e3);
+        assert(e1 == e3);
+    }
+
+    { // Some extents are static, some dynamic
+        extents<int, 3, dynamic_extent> e1{1};
+        extents<long, dynamic_extent, 3> e2{2};
+        extents<size_t, 2, dynamic_extent> e3{3};
+        assert(e1 != e2);
+        assert(e2 == e3);
+        assert(e1 != e2);
+    }
+
+    { // All extents are dynamic
+        dextents<int, 2> e1{1, 2};
+        dextents<long, 2> e2{1, 2};
+        dextents<size_t, 2> e3{1, 3};
+        assert(e1 == e2);
+        assert(e2 != e3);
+        assert(e1 != e3);
+    }
+}
+
+constexpr bool test() {
+    // check_members<short>(); // FIXME Definitely a bug.
+    check_members<int, 1, 2, 3>();
+    check_members<unsigned long long, dynamic_extent, 4, 5>();
+    // check_members<short, dynamic_extent, dynamic_extent, 6>(); // FIXME Bug in array/span constructor?
+    check_members<unsigned char, dynamic_extent, dynamic_extent, dynamic_extent>();
+    check_construction_from_other_extents();
+    check_construction_from_extents_pack();
+    check_construction_from_array_and_span();
+    check_equality_operator();
+    return true;
+}
+
+template <class T, size_t ExpectedRank>
+constexpr bool all_extents_dynamic = false;
+
+template <class IndexType, size_t... Extents, size_t ExpectedRank>
+constexpr bool all_extents_dynamic<extents<IndexType, Extents...>, ExpectedRank> =
+    ((Extents == dynamic_extent) && ...) && (sizeof...(Extents) == ExpectedRank);
+
+template <class... Args>
+concept CanDeduceExtents = requires(Args&&... args) { extents{forward<Args>(args)...}; };
+
+// Check deduction guide
+using DG = decltype(extents{'1', 2, 3u, 4ll, ConvertibleToInt<size_t>{}});
+static_assert(all_extents_dynamic<DG, 5>);
+static_assert(same_as<DG::index_type, size_t>);
+static_assert(!CanDeduceExtents<int, long, short, NonConvertibleToAnything>);
+
+// Check dextents
+static_assert(all_extents_dynamic<dextents<signed char, 0>, 0>);
+static_assert(all_extents_dynamic<dextents<unsigned short, 2>, 2>);
+static_assert(all_extents_dynamic<dextents<int, 3>, 3>);
+static_assert(all_extents_dynamic<dextents<unsigned long, 5>, 5>);
+
+int main() {
+    static_assert(test());
+    test();
+}