Index: libcxx/include/__utility/declval.h
===================================================================
--- libcxx/include/__utility/declval.h
+++ libcxx/include/__utility/declval.h
@@ -27,7 +27,12 @@
 _LIBCPP_SUPPRESS_DEPRECATED_POP
 
 template <class _Tp>
-decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT;
+inline constexpr bool __false = false;
+
+template <class _Tp>
+decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
+    static_assert(__false<_Tp>, "Calling declval is ill-formed, see [declval]/2.");
+}
 
 _LIBCPP_END_NAMESPACE_STD