library test;

T f<T>() => throw '';

class A {
  C operator +(F v) => throw '';
  C operator -(int i) => throw '';
}

class B extends A {
  D operator +(E v) => throw '';
  D operator -(int i) => throw '';
}

class C extends B {}

class D extends C {}

class E extends D {}

class F extends E {}

class Index {}

class Test {
  B operator [](Index i) => throw '';
  void operator []=(Index i, A v) {}
  void test() {}
}

class Test2 {
  B? operator [](Index i) => throw '';
  void operator []=(Index i, A? v) {}
  void test() {}
}

main() {}