// @dart = 2.9
extension<R, T> on R Function(T) {
  Type get returnType => R;
  Type get parameterType => T;
}

class Class<T extends Class<T>> {}

class Subclass extends Class<Subclass> {}

extension<T extends Class<T>> on dynamic Function<S extends T>(T, S) {
  Type get parameterType => T;
}

main() {}