class A {}
class B extends A { void foo() }

/// Unsound (covariant argument types)
class C {
    void m(A a) { }
}
class D extends C {
    void m(B b) { b.foo(); }
}
main: ((C)new D()).m(new A())












class A {}
class B extends A { void foo() }

/// Unsound (contravariant return types)
class C {
    B m() { return new B(); }
}
class D extends C {
    A m() { return new A(); }
}
main: ((C)new D()).m().foo();












class A {}
class B extends A { void foo() }

/// Unsound (depth subtyping on mutable fields)
class C { A f; }
class D extends C { B f; }
main: {
    D d = new D();
    ((C)d).f = new A();
    d.f.foo();
}












class A {}
class B extends A { void foo() }

/// Unsound (depth subtyping on mutable fields)
class C { B f; }
class D extends C { A f; }
main: {
    D d = new D();
    d.f = new A();
    ((C)d).f.foo();
}