class Super {
  void method(num a) {}
  num operator [](num a) => 0;
  void operator []=(num a, num b) {}
  void set setter(num a) {}
}

class Super1 {
  num? operator [](num a) => null;
  void operator []=(num a, num b) {}
}

class Class extends Super {
  void method(covariant int a);
  num operator [](covariant int a);
  void operator []=(covariant int a, covariant int b);
  void set setter(covariant int a);
}

class Class1 extends Super1 {
  num? operator [](covariant int a);
  void operator []=(covariant int a, covariant int b);
}

class Subclass extends Class {
  void method(int a) {}
  num operator [](covariant int a) {}
  void operator []=(covariant int a, covariant int b) {}
  void set setter(int a) {}
}

class Subclass1 extends Class1 {
  num? operator [](covariant int a) {}
  void operator []=(covariant int a, covariant int b) {}
}

mixin Mixin on Class {
  void method(int a) {}
  num operator [](covariant int a) {}
  void operator []=(covariant int a, covariant int b) {}
  void set setter(int a) {}
}
mixin Mixin1 on Class1 {
  void operator []=(covariant int a, covariant int b) {}
}