contravariant argument types: wanna?

[dsimcha]

== Quote from Andrei Alexandrescu (SeeWebsiteForEmail at erdani.org)'s article
> 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

If I understand covariance and contravariance right, this seems like a no-brainer
and probably easy to implement if you already have a working familiarity with the
DMD codebase.  What would there be to the implementation besides:

1.  Making the compiler accept class hierarchies for which contravariant arguments
are declared, and
2.  Inserting the necessary implicit upcasts.