`<variant>`: Make `visit<R>` correctly handle conversions to non-movable types (#2971)

Co-authored-by: Stephan T. Lavavej <stl@microsoft.com>
Co-authored-by: Casey Carter <Casey@Carter.net>

stl/inc/variant

@@ -35,8 +35,8 @@ template <class _First, class... _Rest>
 struct _All_same<_First, _Rest...> : bool_constant<conjunction_v<is_same<_First, _Rest>...>> {}; // variadic is_same
 
 template <class _To, class... _From>
-struct _Convertible_from_all : bool_constant<conjunction_v<is_convertible<_From, _To>...>> {
+struct _Convertible_from_all : bool_constant<conjunction_v<_Invoke_convertible<_From, _To>...>> {
-    // variadic is_convertible
+    // variadic _Invoke_convertible
 };
 
 template <class...>

tests/std/tests/P0088R3_variant/test.cpp

@@ -6971,6 +6971,28 @@ namespace msvc {
                 using R = immobile_data;
                 assert(std::visit<R>(std::identity{}, std::variant<int, short>{13}).x == 13);
                 assert(std::visit<R>(std::identity{}, std::variant<int, short>{short{42}}).x == 42);
+
+                // Verify that conversions to an object that can't be copied/moved are correctly handled
+                struct convertible_to_immobile_one {
+                    operator immobile_data() const {
+                        return immobile_data{1729};
+                    }
+                };
+
+                struct convertible_to_immobile_other {
+                    operator immobile_data() const {
+                        return immobile_data{1138};
+                    }
+                };
+
+                using VarTestConv = std::variant<convertible_to_immobile_one, convertible_to_immobile_other>;
+#if defined(__clang__) || defined(__EDG__) // TRANSITION, DevCom-10112408
+                assert(std::visit<R>(std::identity{}, VarTestConv{convertible_to_immobile_one{}}).x == 1729);
+                assert(std::visit<R>(std::identity{}, VarTestConv{convertible_to_immobile_other{}}).x == 1138);
+#endif // TRANSITION, DevCom-10112408
+                auto immobile_converter = [](auto src) -> immobile_data { return src; };
+                assert(std::visit<R>(immobile_converter, VarTestConv{convertible_to_immobile_one{}}).x == 1729);
+                assert(std::visit<R>(immobile_converter, VarTestConv{convertible_to_immobile_other{}}).x == 1138);
             }
             {
                 // Verify that a returned object is not copied/moved/modified