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>
+decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
+    static_assert(_False, "Calling declval is ill-formed, see [declval]/2.");
+}
 
 _LIBCPP_END_NAMESPACE_STD