import 'dart:async';

abstract class A {
  T foo<T>(B<T> b);
}

abstract class D<W> implements A {}

class B<X> {
  B(X x);
}

class C<Y> {
  C(this.bar);
  final Bar<FutureOr<Y>, D<Y>> bar;
}

final baz = C<int>((a) {
  return a.foo(B(Future.value(0)));
});
main() {}
typedef Bar<V, U extends A> = V Function(U);