library test;

import 'dart:async';

T f<T>() => null;

class D<T> {}

class E<T> extends D<T> {}

class B<T> {
  D<T> operator [](E<T> x) => null;
}

class C<U> extends B<Future<U>> {
  E<Future<U>> operator [](Object x) => null;
  void h() {}
}

main() {}