* If you have advanced custom logic to access each route's `route.response_class`, or the `router.default_response_class`, or the `app.default_response_class`: the default value for `response_class` in `APIRoute` and for `default_response_class` in `APIRouter` and `FastAPI` is now a `DefaultPlaceholder` used internally to handle and solve default values and overrides. The actual response class inside the `DefaultPlaceholder` is available at `route.response_class.value`....