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

Author: philnik777

libcxx/include/variant

@@ -1182,13 +1182,21 @@ public:
   _LIBCPP_HIDE_FROM_ABI constexpr variant(const variant&) = default;
   _LIBCPP_HIDE_FROM_ABI constexpr variant(variant&&)      = default;
 
-  template < class _Arg,
-             enable_if_t<!is_same_v<__remove_cvref_t<_Arg>, variant>, int>        = 0,
-             enable_if_t<!__is_inplace_type<__remove_cvref_t<_Arg>>::value, int>  = 0,
-             enable_if_t<!__is_inplace_index<__remove_cvref_t<_Arg>>::value, int> = 0,
-             class _Tp  = __variant_detail::__best_match_t<_Arg, _Types...>,
-             size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
-             enable_if_t<is_constructible_v<_Tp, _Arg>, int> = 0>
+  template <class _Arg,
+            bool = !is_same_v<__remove_cvref_t<_Arg>, variant> && !__is_inplace_type<__remove_cvref_t<_Arg>>::value &&
+                   !__is_inplace_index<__remove_cvref_t<_Arg>>::value,
+            class = void>
+  struct __arg_overload_type {};
+
+  template <class _Arg>
+  struct __arg_overload_type<_Arg, true, __void_t<__variant_detail::__best_match_t<_Arg, _Types...>>> {
+    using type _LIBCPP_NODEBUG = __variant_detail::__best_match_t<_Arg, _Types...>;
+  };
+
+  template <class _Arg,
+            class _Tp  = typename __arg_overload_type<_Arg>::type,
+            size_t _Ip = __find_detail::__find_unambiguous_index_sfinae<_Tp, _Types...>::value,
+            enable_if_t<is_constructible_v<_Tp, _Arg>, int> = 0>
   _LIBCPP_HIDE_FROM_ABI constexpr variant(_Arg&& __arg) noexcept(is_nothrow_constructible_v<_Tp, _Arg>)
       : __impl_(in_place_index<_Ip>, std::forward<_Arg>(__arg)) {}
 

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
+    struct Matcher {
+      Matcher() {}
+      Matcher(std::variant<ConvertibleFromAny>) {}
+    };
+
+    Matcher vec;
+    [[maybe_unused]] Matcher m = std::move(vec);
+  }
+
   return 0;
 }