contravariant argument types: wanna?

[Andrei Alexandrescu]

Hello,

Today, overriding functions have covariant return types:

class A {
    A clone();
}

class B : A {
    B clone(); // fine, overrides A.clone
}

That is entirely principled and cool. Now the entire story is that 
overriding function may have not only covariant return types, but also 
contravariant argument types:

class A {
    A fun(B);
}

class B : A {
    B fun(A); // fine (in theory), overrides A.fun
}

Today D does not support contravariant arguments, but Walter told me 
once he'd be quite willing to implement them. It is definitely the right 
thing to do, but Walter would want to see a compelling example before 
getting to work.

Is there interest in contravariant argument types? If so, do you know of 
a killer example?


Thanks,

Andrei