diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/less.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/less.pass.cpp
@@ -0,0 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <string_view>
+
+// template<class charT, class traits>
+//   constexpr bool operator<(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs);
+// (plus "sufficient additional overloads" to make implicit conversions work as intended)
+
+#include <string_view>
+#include <cassert>
+#include <string>
+
+#include "test_macros.h"
+#include "constexpr_char_traits.h"
+#include "make_string.h"
+
+template<class T>
+struct ConvertibleTo {
+    T t_;
+    TEST_CONSTEXPR explicit ConvertibleTo(T t) : t_(t) {}
+    TEST_CONSTEXPR operator T() const {
+        return t_;
+    }
+};
+
+template<class SV>
+TEST_CONSTEXPR_CXX14 bool test()
+{
+    typedef typename SV::value_type CharT;
+    typedef typename SV::traits_type Traits;
+
+    // Test the behavior of the operator, both with and without implicit conversions.
+    SV v[] = {
+        SV(MAKE_CSTRING(CharT, "")),
+        SV(MAKE_CSTRING(CharT, "abc")),
+        SV(MAKE_CSTRING(CharT, "abcdef")),
+        SV(MAKE_CSTRING(CharT, "acb")),
+    };
+    for (int i = 0; i < 4; ++i) {
+        for (int j = 0; j < 4; ++j) {
+            // See http://eel.is/c++draft/string.view#tab:string.view.comparison.overloads
+            bool expected = (i < j);
+            assert((v[i] < v[j]) == expected);
+            assert((v[i].data() < v[j]) == expected);
+            assert((v[i] < v[j].data()) == expected);
+            assert((ConvertibleTo<SV>(v[i]) < v[j]) == expected);
+            assert((v[i] < ConvertibleTo<SV>(v[j])) == expected);
+
+            if (!TEST_IS_CONSTANT_EVALUATED) {
+                // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+                assert((std::basic_string<CharT, Traits>(v[i]) < v[j]) == expected);
+                assert((v[i] < std::basic_string<CharT, Traits>(v[j])) == expected);
+            }
+        }
+    }
+
+    // Test its behavior with embedded null bytes.
+    SV abc = SV(MAKE_CSTRING(CharT, "abc"));
+    SV abc0def = SV(MAKE_CSTRING(CharT, "abc\0def"), 7);
+    SV abcdef = SV(MAKE_CSTRING(CharT, "abcdef"));
+    assert((abc < abc0def) == true);
+    assert((abc < abcdef) == true);
+    assert((abc0def < abc) == false);
+    assert((abc0def < abcdef) == true);
+    assert((abcdef < abc) == false);
+    assert((abcdef < abc0def) == false);
+
+    assert((abc.data() < abc0def) == true);
+    assert((abc0def < abc.data()) == false);
+
+    if (!TEST_IS_CONSTANT_EVALUATED) {
+        // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+        assert((std::basic_string<CharT, Traits>(abc) < abc0def) == true);
+        assert((abc0def < std::basic_string<CharT, Traits>(abc)) == false);
+    }
+
+    return true;
+}
+
+int main(int, char**)
+{
+    test<std::string_view>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    test<std::wstring_view>();
+#endif
+#if TEST_STD_VER >= 11
+    test<std::u16string_view>();
+    test<std::u32string_view>();
+#endif
+#if TEST_STD_VER > 14
+    static_assert(test<std::string_view>(), "");
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    static_assert(test<std::wstring_view>(), "");
+#endif
+    static_assert(test<std::u16string_view>(), "");
+    static_assert(test<std::u32string_view>(), "");
+#endif
+
+#if TEST_STD_VER > 11
+    test<std::basic_string_view<char, constexpr_char_traits<char>>>();
+    static_assert(test<std::basic_string_view<char, constexpr_char_traits<char>>>(), "");
+#endif
+
+#if TEST_STD_VER > 17
+    test<std::u8string_view>();
+    static_assert(test<std::u8string_view>());
+#endif
+
+    return 0;
+}
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/less_equal.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/less_equal.pass.cpp
@@ -0,0 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <string_view>
+
+// template<class charT, class traits>
+//   constexpr bool operator<=(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs);
+// (plus "sufficient additional overloads" to make implicit conversions work as intended)
+
+#include <string_view>
+#include <cassert>
+#include <string>
+
+#include "test_macros.h"
+#include "constexpr_char_traits.h"
+#include "make_string.h"
+
+template<class T>
+struct ConvertibleTo {
+    T t_;
+    TEST_CONSTEXPR explicit ConvertibleTo(T t) : t_(t) {}
+    TEST_CONSTEXPR operator T() const {
+        return t_;
+    }
+};
+
+template<class SV>
+TEST_CONSTEXPR_CXX14 bool test()
+{
+    typedef typename SV::value_type CharT;
+    typedef typename SV::traits_type Traits;
+
+    // Test the behavior of the operator, both with and without implicit conversions.
+    SV v[] = {
+        SV(MAKE_CSTRING(CharT, "")),
+        SV(MAKE_CSTRING(CharT, "abc")),
+        SV(MAKE_CSTRING(CharT, "abcdef")),
+        SV(MAKE_CSTRING(CharT, "acb")),
+    };
+    for (int i = 0; i < 4; ++i) {
+        for (int j = 0; j < 4; ++j) {
+            // See http://eel.is/c++draft/string.view#tab:string.view.comparison.overloads
+            bool expected = (i <= j);
+            assert((v[i] <= v[j]) == expected);
+            assert((v[i].data() <= v[j]) == expected);
+            assert((v[i] <= v[j].data()) == expected);
+            assert((ConvertibleTo<SV>(v[i]) <= v[j]) == expected);
+            assert((v[i] <= ConvertibleTo<SV>(v[j])) == expected);
+
+            if (!TEST_IS_CONSTANT_EVALUATED) {
+                // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+                assert((std::basic_string<CharT, Traits>(v[i]) <= v[j]) == expected);
+                assert((v[i] <= std::basic_string<CharT, Traits>(v[j])) == expected);
+            }
+        }
+    }
+
+    // Test its behavior with embedded null bytes.
+    SV abc = SV(MAKE_CSTRING(CharT, "abc"));
+    SV abc0def = SV(MAKE_CSTRING(CharT, "abc\0def"), 7);
+    SV abcdef = SV(MAKE_CSTRING(CharT, "abcdef"));
+    assert((abc <= abc0def) == true);
+    assert((abc <= abcdef) == true);
+    assert((abc0def <= abc) == false);
+    assert((abc0def <= abcdef) == true);
+    assert((abcdef <= abc) == false);
+    assert((abcdef <= abc0def) == false);
+
+    assert((abc.data() <= abc0def) == true);
+    assert((abc0def <= abc.data()) == false);
+
+    if (!TEST_IS_CONSTANT_EVALUATED) {
+        // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+        assert((std::basic_string<CharT, Traits>(abc) <= abc0def) == true);
+        assert((abc0def <= std::basic_string<CharT, Traits>(abc)) == false);
+    }
+
+    return true;
+}
+
+int main(int, char**)
+{
+    test<std::string_view>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    test<std::wstring_view>();
+#endif
+#if TEST_STD_VER >= 11
+    test<std::u16string_view>();
+    test<std::u32string_view>();
+#endif
+#if TEST_STD_VER > 14
+    static_assert(test<std::string_view>(), "");
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    static_assert(test<std::wstring_view>(), "");
+#endif
+    static_assert(test<std::u16string_view>(), "");
+    static_assert(test<std::u32string_view>(), "");
+#endif
+
+#if TEST_STD_VER > 11
+    test<std::basic_string_view<char, constexpr_char_traits<char>>>();
+    static_assert(test<std::basic_string_view<char, constexpr_char_traits<char>>>(), "");
+#endif
+
+#if TEST_STD_VER > 17
+    test<std::u8string_view>();
+    static_assert(test<std::u8string_view>());
+#endif
+
+    return 0;
+}
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/not_equal.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/not_equal.pass.cpp
@@ -0,0 +1,116 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <string_view>
+
+// template<class charT, class traits>
+//   constexpr bool operator!=(basic_string_view<charT, traits> lhs, basic_string_view<charT, traits> rhs);
+// (plus "sufficient additional overloads" to make implicit conversions work as intended)
+
+#include <string_view>
+#include <cassert>
+#include <string>
+
+#include "test_macros.h"
+#include "constexpr_char_traits.h"
+#include "make_string.h"
+
+template<class T>
+struct ConvertibleTo {
+    T t_;
+    TEST_CONSTEXPR explicit ConvertibleTo(T t) : t_(t) {}
+    TEST_CONSTEXPR operator T() const {
+        return t_;
+    }
+};
+
+template<class SV>
+TEST_CONSTEXPR_CXX14 bool test()
+{
+    typedef typename SV::value_type CharT;
+    typedef typename SV::traits_type Traits;
+
+    // Test the behavior of the operator, both with and without implicit conversions.
+    SV v[] = {
+        SV(MAKE_CSTRING(CharT, "")),
+        SV(MAKE_CSTRING(CharT, "abc")),
+        SV(MAKE_CSTRING(CharT, "abcdef")),
+        SV(MAKE_CSTRING(CharT, "acb")),
+    };
+    for (int i = 0; i < 4; ++i) {
+        for (int j = 0; j < 4; ++j) {
+            // See http://eel.is/c++draft/string.view#tab:string.view.comparison.overloads
+            bool expected = (i != j);
+            assert((v[i] != v[j]) == expected);
+            assert((v[i].data() != v[j]) == expected);
+            assert((v[i] != v[j].data()) == expected);
+            assert((ConvertibleTo<SV>(v[i]) != v[j]) == expected);
+            assert((v[i] != ConvertibleTo<SV>(v[j])) == expected);
+
+            if (!TEST_IS_CONSTANT_EVALUATED) {
+                // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+                assert((std::basic_string<CharT, Traits>(v[i]) != v[j]) == expected);
+                assert((v[i] != std::basic_string<CharT, Traits>(v[j])) == expected);
+            }
+        }
+    }
+
+    // Test its behavior with embedded null bytes.
+    SV abc = SV(MAKE_CSTRING(CharT, "abc"));
+    SV abc0def = SV(MAKE_CSTRING(CharT, "abc\0def"), 7);
+    SV abcdef = SV(MAKE_CSTRING(CharT, "abcdef"));
+    assert((abc != abc0def) == true);
+    assert((abc != abcdef) == true);
+    assert((abc0def != abc) == true);
+    assert((abc0def != abcdef) == true);
+    assert((abcdef != abc) == true);
+    assert((abcdef != abc0def) == true);
+
+    assert((abc.data() != abc0def) == true);
+    assert((abc0def != abc.data()) == true);
+
+    if (!TEST_IS_CONSTANT_EVALUATED) {
+        // TODO FIXME: once P0980 "Making std::string constexpr" is implemented
+        assert((std::basic_string<CharT, Traits>(abc) != abc0def) == true);
+        assert((abc0def != std::basic_string<CharT, Traits>(abc)) == true);
+    }
+
+    return true;
+}
+
+int main(int, char**)
+{
+    test<std::string_view>();
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    test<std::wstring_view>();
+#endif
+#if TEST_STD_VER >= 11
+    test<std::u16string_view>();
+    test<std::u32string_view>();
+#endif
+#if TEST_STD_VER > 14
+    static_assert(test<std::string_view>(), "");
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+    static_assert(test<std::wstring_view>(), "");
+#endif
+    static_assert(test<std::u16string_view>(), "");
+    static_assert(test<std::u32string_view>(), "");
+#endif
+
+#if TEST_STD_VER > 11
+    test<std::basic_string_view<char, constexpr_char_traits<char>>>();
+    static_assert(test<std::basic_string_view<char, constexpr_char_traits<char>>>(), "");
+#endif
+
+#if TEST_STD_VER > 17
+    test<std::u8string_view>();
+    static_assert(test<std::u8string_view>());
+#endif
+
+    return 0;
+}
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opeq.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opeq.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opeq.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opeq.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/opeq.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/opeq.string_view.string_view.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opge.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opge.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opge.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opge.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/opge.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/opge.string_view.string_view.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opgt.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opgt.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opgt.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opgt.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/opgt.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/opgt.string_view.string_view.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/ople.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/ople.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/ople.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/ople.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/ople.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/ople.string_view.string_view.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/oplt.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/oplt.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/oplt.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/oplt.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/oplt.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/oplt.string_view.string_view.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opne.string_view.pointer.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opne.string_view.pointer.pass.cpp
diff --git a/libcxx/test/std/strings/string.view/string.view.comparison/opne.string_view.string.pass.cpp b/libcxx/test/std/strings/string.view/string.view.comparison/opne.string_view.string.pass.cpp
diff --git a/...test/std/strings/string.view/string.view.comparison/opne.string_view.string_view.pass.cpp b/...test/std/strings/string.view/string.view.comparison/opne.string_view.string_view.pass.cpp
diff --git a/libcxx/test/support/make_string.h b/libcxx/test/support/make_string.h
@@ -11,69 +11,92 @@
 
 #include "test_macros.h"
 
-#if TEST_STD_VER < 11
-#error This header requires C++11 or greater
-#endif
-
 #include <string>
 #include <string_view>
 
+#ifndef TEST_HAS_NO_WIDE_CHARACTERS
+# define MKSTR_WCHAR_ONLY(...) __VA_ARGS__
+# define MKSTR_AS_WCHAR_LITERAL(x) TEST_CONCAT(L, x), sizeof(TEST_CONCAT(L, x)) / sizeof(wchar_t) - 1
+#else
+# define MKSTR_WCHAR_ONLY(...)
+#endif
+
 #if TEST_STD_VER > 17 && defined(__cpp_char8_t)
-#define CHAR8_ONLY(x) x,
+#define MKSTR_CHAR8_ONLY(...) __VA_ARGS__
+#define MKSTR_AS_U8_LITERAL(x) TEST_CONCAT(u8, x), sizeof(TEST_CONCAT(u8, x)) / sizeof(char8_t) - 1
 #else
-#define CHAR8_ONLY(x)
+#define MKSTR_CHAR8_ONLY(...)
 #endif
 
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-# define IF_WIDE_CHARACTERS(...) __VA_ARGS__
+#if TEST_STD_VER >= 11
+#define MKSTR_CXX11_ONLY(...) __VA_ARGS__
+#define MKSTR_AS_U16_LITERAL(x) TEST_CONCAT(u, x), sizeof(TEST_CONCAT(u, x)) / sizeof(char16_t) - 1
+#define MKSTR_AS_U32_LITERAL(x) TEST_CONCAT(U, x), sizeof(TEST_CONCAT(U, x)) / sizeof(char32_t) - 1
 #else
-# define IF_WIDE_CHARACTERS(...) /* nothing */
+#define MKSTR_CXX11_ONLY(...)
 #endif
 
-#define MKSTR(Str)                                                             \
-  {                                                                            \
-    Str, IF_WIDE_CHARACTERS(TEST_CONCAT(L, Str),)                              \
-        CHAR8_ONLY(TEST_CONCAT(u8, Str)) TEST_CONCAT(u, Str),                  \
-        TEST_CONCAT(U, Str)                                                    \
-  }
+#define MKSTR(Str) MultiStringType(                \
+    MKSTR_WCHAR_ONLY(MKSTR_AS_WCHAR_LITERAL(Str),) \
+    MKSTR_CHAR8_ONLY(MKSTR_AS_U8_LITERAL(Str),)    \
+    MKSTR_CXX11_ONLY(MKSTR_AS_U16_LITERAL(Str),    \
+                     MKSTR_AS_U32_LITERAL(Str),)   \
+    Str, sizeof(Str) - 1                           \
+  )
+
+#define MKSTR_LEN(CharT, Str) MKSTR(Str).length((const CharT*)0)
 
 struct MultiStringType {
-  const char* s;
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  const wchar_t* w;
-#endif
-#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
-  const char8_t* u8;
-#endif
-  const char16_t* u16;
-  const char32_t* u32;
+  MKSTR_WCHAR_ONLY(const wchar_t* w_; size_t wn_; )
+  MKSTR_CHAR8_ONLY(const char8_t* u8_; size_t u8n_; )
+  MKSTR_CXX11_ONLY(const char16_t* u16_; size_t u16n_; )
+  MKSTR_CXX11_ONLY(const char32_t* u32_; size_t u32n_; )
+  const char* s_; size_t sn_;
 
-  constexpr operator const char*() const { return s; }
-#ifndef TEST_HAS_NO_WIDE_CHARACTERS
-  constexpr operator const wchar_t*() const { return w; }
-#endif
-#if TEST_STD_VER > 17 && defined(__cpp_char8_t)
-  constexpr operator const char8_t*() const { return u8; }
-#endif
-  constexpr operator const char16_t*() const { return u16; }
-  constexpr operator const char32_t*() const { return u32; }
+  TEST_CONSTEXPR MultiStringType(
+      MKSTR_WCHAR_ONLY(const wchar_t *w, size_t wn,)
+      MKSTR_CHAR8_ONLY(const char8_t *u8, size_t u8n,)
+      MKSTR_CXX11_ONLY(const char16_t *u16, size_t u16n,)
+      MKSTR_CXX11_ONLY(const char32_t *u32, size_t u32n,)
+      const char *s, size_t sn)
+    : MKSTR_WCHAR_ONLY(w_(w), wn_(wn),)
+      MKSTR_CHAR8_ONLY(u8_(u8), u8n_(u8n),)
+      MKSTR_CXX11_ONLY(u16_(u16), u16n_(u16n),)
+      MKSTR_CXX11_ONLY(u32_(u32), u32n_(u32n),)
+      s_(s), sn_(sn) {}
+
+  TEST_CONSTEXPR const char *as_ptr(const char*) const { return s_; }
+  MKSTR_WCHAR_ONLY(TEST_CONSTEXPR const wchar_t *as_ptr(const wchar_t*) const { return w_; })
+  MKSTR_CHAR8_ONLY(constexpr const char8_t *as_ptr(const char8_t*) const { return u8_; })
+  MKSTR_CXX11_ONLY(constexpr const char16_t *as_ptr(const char16_t*) const { return u16_; })
+  MKSTR_CXX11_ONLY(constexpr const char32_t *as_ptr(const char32_t*) const { return u32_; })
+
+  TEST_CONSTEXPR size_t length(const char*) const { return sn_; }
+  MKSTR_WCHAR_ONLY(TEST_CONSTEXPR size_t length(const wchar_t*) const { return wn_; })
+  MKSTR_CHAR8_ONLY(constexpr size_t length(const char8_t*) const { return u8n_; })
+  MKSTR_CXX11_ONLY(constexpr size_t length(const char16_t*) const { return u16n_; })
+  MKSTR_CXX11_ONLY(constexpr size_t length(const char32_t*) const { return u32n_; })
+
+  // These implicit conversions are used by some tests. TODO: maybe eliminate them?
+  TEST_CONSTEXPR operator const char*() const { return s_; }
+  MKSTR_WCHAR_ONLY(TEST_CONSTEXPR operator const wchar_t*() const { return w_; })
+  MKSTR_CHAR8_ONLY(constexpr operator const char8_t*() const { return u8_; })
+  MKSTR_CXX11_ONLY(constexpr operator const char16_t*() const { return u16_; })
+  MKSTR_CXX11_ONLY(constexpr operator const char32_t*() const { return u32_; })
 };
 
-// Helper to convert a const char* string to a basic_string<CharT>.
+// Helper to convert a const char* string to a const CharT*.
 // This helper is used in unit tests to make them generic. The input should be
 // valid ASCII which means the input is also valid UTF-8.
+#define MAKE_CSTRING(CharT, Str)                                               \
+  MKSTR(Str).as_ptr((const CharT*)0)
+
+// Like MAKE_CSTRING but makes a basic_string<CharT>. Embedded nulls are OK.
 #define MAKE_STRING(CharT, Str)                                                \
-  std::basic_string<CharT> {                                                   \
-    static_cast<const CharT*>(MultiStringType MKSTR(Str))                      \
-  }
+  std::basic_string<CharT>(MAKE_CSTRING(CharT, Str), MKSTR_LEN(CharT, Str))
 
+// Like MAKE_CSTRING but makes a basic_string_view<CharT>. Embedded nulls are OK.
 #define MAKE_STRING_VIEW(CharT, Str)                                           \
-  std::basic_string_view<CharT> {                                              \
-    static_cast<const CharT*>(MultiStringType MKSTR(Str))                      \
-  }
-
-// Like MAKE_STRING but converts to a const CharT*.
-#define MAKE_CSTRING(CharT, Str)                                               \
-  static_cast<const CharT*>(MultiStringType MKSTR(Str))
+  std::basic_string_view<CharT>(MAKE_CSTRING(CharT, Str), MKSTR_LEN(CharT, Str))
 
 #endif
diff --git a/libcxx/test/support/test_macros.h b/libcxx/test/support/test_macros.h
@@ -146,9 +146,11 @@
 #endif
 
 #if defined(__cpp_lib_is_constant_evaluated) && __cpp_lib_is_constant_evaluated >= 201811L
-#  define TEST_IS_CONSTANT_EVALUATED std::is_constant_evaluated()
+# define TEST_IS_CONSTANT_EVALUATED std::is_constant_evaluated()
+#elif __has_builtin(__builtin_is_constant_evaluated)
+# define TEST_IS_CONSTANT_EVALUATED __builtin_is_constant_evaluated()
 #else
-#  define TEST_IS_CONSTANT_EVALUATED false
+# define TEST_IS_CONSTANT_EVALUATED false
 #endif
 
 #if TEST_STD_VER >= 14