class A {}

class B extends A {}

class C {
  void set x(A value) {}
  B get y => null;
}

class D extends C {
  void set x(value) {}
  get y => null;
}

class E extends D {
  void set x(A value) {}
  B get y => null;
}

class F extends D {
  void set x(B value) {}
  A get y => null;
}

main() {}