Index: libcxx/include/__utility/declval.h
===================================================================
--- libcxx/include/__utility/declval.h
+++ libcxx/include/__utility/declval.h
@@ -26,8 +26,10 @@
 _Tp __declval(long);
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
-template <class _Tp>
-decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT;
+template <class _Tp, bool _False = false>
+_LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
+  static_assert(_False, "Calling declval is ill-formed, see [declval]/2.");
+}
 
 _LIBCPP_END_NAMESPACE_STD
 
Index: libcxx/test/std/input.output/iostream.format/output.streams/ostream/deleted_output_functions.verify.cpp
===================================================================
--- libcxx/test/std/input.output/iostream.format/output.streams/ostream/deleted_output_functions.verify.cpp
+++ libcxx/test/std/input.output/iostream.format/output.streams/ostream/deleted_output_functions.verify.cpp
@@ -20,36 +20,43 @@
   std::ostringstream s;
 
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
-
-  s << wchar_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<wchar_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<const wchar_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  wchar_t w_s[]       = L"test";
+  const wchar_t* w_cs = L"test";
+  s << wchar_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << w_s;       // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << w_cs;      // expected-error {{overload resolution selected deleted operator '<<'}}
 
   std::wostringstream sw;
 #  ifndef TEST_HAS_NO_CHAR8_T
-  sw << char8_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<char8_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<const char8_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  char8_t c8_s[]       = u8"test";
+  const char8_t* c8_cs = u8"test";
+  sw << char8_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c8_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c8_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
 #  endif
 
-  sw << char16_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<char16_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<const char16_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << char32_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<char32_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  sw << std::declval<const char32_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  char16_t c16_s[]       = u"test";
+  const char16_t* c16_cs = u"test";
+  char32_t c32_s[]       = U"test";
+  const char32_t* c32_cs = U"test";
+  sw << char16_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c16_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c16_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << char32_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c32_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  sw << c32_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
 
 #endif // TEST_HAS_NO_WIDE_CHARACTERS
 
 #ifndef TEST_HAS_NO_CHAR8_T
-  s << char8_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<char8_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<const char8_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << char8_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c8_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c8_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
 #endif
-  s << char16_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<char16_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<const char16_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << char32_t();                      // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<char32_t*>();       // expected-error {{overload resolution selected deleted operator '<<'}}
-  s << std::declval<const char32_t*>(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << char16_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c16_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c16_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << char32_t(); // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c32_s;      // expected-error {{overload resolution selected deleted operator '<<'}}
+  s << c32_cs;     // expected-error {{overload resolution selected deleted operator '<<'}}
 }
Index: libcxx/test/std/utilities/utility/declval/declval.verify.cpp
===================================================================
--- /dev/null
+++ libcxx/test/std/utilities/utility/declval/declval.verify.cpp
@@ -0,0 +1,23 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// <utility>
+
+// template <class T> typename add_rvalue_reference<T>::type declval() noexcept;
+
+#include <utility>
+
+int main() {
+  if (false) {
+    int i = std::declval<int>(); // expected-warning@*:* {{code will never be executed}}
+    (void)i;
+  }
+
+  return 0;
+}
+// expected-error-re@*:* {{{{(static_assert|static assertion)}} failed{{.*}}Calling declval is ill-formed, see [declval]/2.}}