library test;

typedef void F<T>(T x);

class B<T> {
  B<T> operator +(B<T> other) => null;
}

class C<T> {
  B<F<T>> operator [](int i) => null;
  void operator []=(int i, B<F<T>> x) {}
}

void test(C<num> c) {}
main() {}