[libcxx-commits] [PATCH] D145376: [libc++] add declval failure assertion for instantiation

[Igor Zhukov via Phabricator via libcxx-commits]

fsb4000 created this revision.
fsb4000 added reviewers: Mordante, philnik, libc++.
fsb4000 added a project: libc++.
Herald added a project: All.
fsb4000 requested review of this revision.
Herald added a subscriber: libcxx-commits.
Herald added 1 blocking reviewer(s): libc++.

Fixes https://github.com/llvm/llvm-project/issues/61202


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D145376

Files:
  libcxx/include/__utility/declval.h


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
 


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