[libc++] Fix std::variant/std::invoke too eager instantiation

Author: philnik777

libcxx/include/__type_traits/invoke.h

@@ -22,6 +22,7 @@
 #include <__type_traits/is_same.h>
 #include <__type_traits/is_void.h>
 #include <__type_traits/nat.h>
+#include <__type_traits/type_identity.h>
 #include <__type_traits/void_t.h>
 #include <__utility/declval.h>
 #include <__utility/forward.h>
@@ -67,20 +68,20 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if __has_builtin(__builtin_invoke)
 
-template <class... _Args>
-using __invoke_result_t _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));
-
 template <class, class... _Args>
 struct __invoke_result_impl {};
 
 template <class... _Args>
-struct __invoke_result_impl<__void_t<__invoke_result_t<_Args...> >, _Args...> {
-  using type _LIBCPP_NODEBUG = __invoke_result_t<_Args...>;
+struct __invoke_result_impl<__void_t<decltype(__builtin_invoke(std::declval<_Args...>()))>, _Args...> {
+  using type _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args...>()));
 };
 
 template <class... _Args>
 using __invoke_result _LIBCPP_NODEBUG = __invoke_result_impl<void, _Args...>;
 
+template <class... _Args>
+using __invoke_result_t _LIBCPP_NODEBUG = typename __invoke_result<_Args...>::type;
+
 template <class... _Args>
 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __invoke_result_t<_Args...> __invoke(_Args&&... __args)
     _NOEXCEPT_(noexcept(__builtin_invoke(std::forward<_Args>(__args)...))) {

libcxx/test/std/test.pass.cpp

@@ -0,0 +1,24 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include <variant>
+#include <utility>
+
+struct Nullable {
+  template <typename V> Nullable(V);
+};
+
+struct Matcher {
+  Matcher();
+  Matcher(std::variant<Nullable>);
+};
+
+void f() {
+  Matcher vec;
+  Matcher m = std::move(vec);
+}