[libc++] Fix std::variant evaluating template arguments too eagerly (#151028)

This has been reported in llvm/llvm-project#116709 (comment).

Fixes #151328

Author: philnik777

libcxx/include/__type_traits/invoke.h

@@ -67,20 +67,20 @@ _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/utilities/variant/variant.variant/variant.ctor/T.pass.cpp

@@ -173,12 +173,28 @@ void test_vector_bool() {
   assert(std::get<0>(v) == true);
 }
 
+struct ConvertibleFromAny {
+  template <class V>
+  ConvertibleFromAny(V) {}
+};
+
 int main(int, char**) {
   test_T_ctor_basic();
   test_T_ctor_noexcept();
   test_T_ctor_sfinae();
   test_no_narrowing_check_for_class_types();
   test_construction_with_repeated_types();
   test_vector_bool();
+
+  { // Check that the constraints are evaluated lazily (see https://github.com/llvm/llvm-project/issues/151328)
+    struct Matcher {
+      Matcher() {}
+      Matcher(std::variant<ConvertibleFromAny>) {}
+    };
+
+    Matcher vec;
+    [[maybe_unused]] Matcher m = std::move(vec);
+  }
+
   return 0;
 }