library test;

typedef void F<T>(T x);

class B<T> {
  B<T> operator +(B<T> other) => throw '';
}

class C<T> {
  B<F<T>> operator [](int i) => throw '';
  void operator []=(int i, B<F<T>> x) {}
}

class C2<T> {
  B<F<T>>? operator [](int i) => throw '';
  void operator []=(int i, B<F<T>>? x) {}
}

void test(C<num> c, C2<num> c2) {}
main() {}