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diff --git a/libcxx/test/libcxx-03/strings/c.strings/constexpr_memmove.pass.cpp b/libcxx/test/libcxx-03/strings/c.strings/constexpr_memmove.pass.cpp
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index 000000000000..d2ca5a265852
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+++ b/libcxx/test/libcxx-03/strings/c.strings/constexpr_memmove.pass.cpp
@@ -0,0 +1,155 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// _Tp* __constexpr_memmove(_Tp* __dest, _Up* __src, __element_count __n);
+//
+// General tests for __constexpr_memmove.
+//
+// In particular, we try to ensure that __constexpr_memmove behaves like
+// __builtin_memmove as closely as possible. This means that it produces the
+// same effect, but also that it has the same type requirements.
+//
+// __builtin_memmove only requires that the types are TriviallyCopyable
+// (which is interestingly different from both is_trivially_XXX_constructible
+// and is_trivially_XXX_assignable), so we use some funky types to test these
+// corner cases.
+
+#include <__cxx03/__string/constexpr_c_functions.h>
+#include <cassert>
+#include <cstdint>
+#include <type_traits>
+
+#include "test_macros.h"
+
+// The following types are all TriviallyCopyable, but they are not all
+// trivially_{copy,move}_{constructible,assignable}. TriviallyCopyable
+// guarantees that the type is *at least* one of the four, but no more
+// than that.
+struct CopyConstructible {
+  CopyConstructible() = default;
+  int value           = 0;
+
+  CopyConstructible(const CopyConstructible&)            = default;
+  CopyConstructible(CopyConstructible&&)                 = delete;
+  CopyConstructible& operator=(const CopyConstructible&) = delete;
+  CopyConstructible& operator=(CopyConstructible&&)      = delete;
+};
+
+struct MoveConstructible {
+  MoveConstructible() = default;
+  int value           = 0;
+
+  MoveConstructible(const MoveConstructible&)            = delete;
+  MoveConstructible(MoveConstructible&&)                 = default;
+  MoveConstructible& operator=(const MoveConstructible&) = delete;
+  MoveConstructible& operator=(MoveConstructible&&)      = delete;
+};
+
+struct CopyAssignable {
+  CopyAssignable() = default;
+  int value        = 0;
+
+  CopyAssignable(const CopyAssignable&)            = delete;
+  CopyAssignable(CopyAssignable&&)                 = delete;
+  CopyAssignable& operator=(const CopyAssignable&) = default;
+  CopyAssignable& operator=(CopyAssignable&&)      = delete;
+};
+
+struct MoveAssignable {
+  MoveAssignable() = default;
+  int value        = 0;
+
+  MoveAssignable(const MoveAssignable&)            = delete;
+  MoveAssignable(MoveAssignable&&)                 = delete;
+  MoveAssignable& operator=(const MoveAssignable&) = delete;
+  MoveAssignable& operator=(MoveAssignable&&)      = default;
+};
+
+template <class Source, class Dest>
+TEST_CONSTEXPR_CXX14 void test_user_defined_types() {
+  static_assert(std::is_trivially_copyable<Source>::value, "test the test");
+  static_assert(std::is_trivially_copyable<Dest>::value, "test the test");
+
+  // Note that we can't just initialize with an initializer list since some of the types we use here
+  // are not copy-constructible, which is required in pre-C++20 Standards for that syntax to work.
+  Source src[3];
+  src[0].value = 1;
+  src[1].value = 2;
+  src[2].value = 3;
+  Dest dst[3];
+  dst[0].value = 111;
+  dst[1].value = 111;
+  dst[2].value = 111;
+
+  Dest* result = std::__constexpr_memmove(dst, src, std::__element_count(3));
+  assert(result == dst);
+  assert(dst[0].value == 1);
+  assert(dst[1].value == 2);
+  assert(dst[2].value == 3);
+}
+
+template <class Source, class Dest>
+TEST_CONSTEXPR_CXX14 void test_builtin_types() {
+  Source src[3] = {1, 2, 3};
+  Dest dst[3]   = {111, 111, 111};
+
+  Dest* result = std::__constexpr_memmove(dst, src, std::__element_count(3));
+  assert(result == dst);
+  assert(dst[0] == 1);
+  assert(dst[1] == 2);
+  assert(dst[2] == 3);
+}
+
+template <class SourcePtr, class DestPtr, class ObjectType>
+TEST_CONSTEXPR_CXX14 void test_pointer_types() {
+  ObjectType objs[3] = {1, 2, 3};
+
+  SourcePtr src[3] = {objs + 0, objs + 1, objs + 2};
+  DestPtr dst[3]   = {nullptr, nullptr, nullptr};
+
+  DestPtr* result = std::__constexpr_memmove(dst, src, std::__element_count(3));
+  assert(result == dst);
+  assert(dst[0] == objs + 0);
+  assert(dst[1] == objs + 1);
+  assert(dst[2] == objs + 2);
+}
+
+TEST_CONSTEXPR_CXX14 bool test() {
+  test_user_defined_types<CopyConstructible, CopyConstructible>();
+  test_user_defined_types<MoveConstructible, MoveConstructible>();
+  test_user_defined_types<CopyAssignable, CopyAssignable>();
+  test_user_defined_types<MoveAssignable, MoveAssignable>();
+
+  test_builtin_types<char, char>();
+  test_builtin_types<short, short>();
+  test_builtin_types<int, int>();
+  test_builtin_types<long, long>();
+  test_builtin_types<long long, long long>();
+
+  // Cross-type
+  test_builtin_types<std::int16_t, std::uint16_t>();
+  test_builtin_types<std::int16_t, char16_t>();
+  test_builtin_types<std::int32_t, std::uint32_t>();
+  test_builtin_types<std::int32_t, char32_t>();
+
+  test_pointer_types<char*, char*, char>();
+  test_pointer_types<int*, int*, int>();
+  test_pointer_types<long*, long*, long>();
+  test_pointer_types<void*, void*, int>();
+  test_pointer_types<int* const, int*, int>();
+
+  return true;
+}
+
+int main(int, char**) {
+  test();
+#if TEST_STD_VER >= 14
+  static_assert(test(), "");
+#endif
+  return 0;
+}