// @dart = 2.9
class A {}

class B extends A {}

class C {
  void f1(A x) {}
  void f2([A x]) {}
  void f3({A x}) {}
  A f4() {}
}

class D extends C {
  void f1(Object x) {}
  void f2([Object x]) {}
  void f3({Object x}) {}
  B f4() {}
}

class E extends C {
  void f1(B x) {}
  void f2([B x]) {}
  void f3({B x}) {}
  Object f4() {}
}

main() {}