import 'dart:async';

class B<Y> {
  bar(FutureOr<Y> y) {}
}

class A<X> {
  final b = new B<X>();
  foo([FutureOr<X>? x]) {}
}

class C<T> {
  FutureOr<T> baz<X extends FutureOr<T>>(FutureOr<T> x) => x;
}

class D<T> extends C<T> {
  FutureOr<T> baz<X extends FutureOr<T>>(FutureOr<T> x) => x;
}

main() {}