Why is there no parameter contra-variance for overriding?

Question

C++ and Java support return-type covariance when overriding methods.

Neither, however, support contra-variance in parameter types - instead, it translates to overl

Answer 1

Thanks to donroby's and David's answers, I think I understand that the main problem with introducing parameter contra-variance is the integration with the overloading mechanism.

So not only is there a problem with a single override for multiple methods, but also the other way:

class A {
    public void f(String s) {...}
}

class B extends A {
    public void f(String s) {...} // this can override A.f
    public void f(Object o) {...} // with contra-variance, so can this!
}

And now there are two valid overrides for the same method:

A a = new B();
a.f(); // which f is called?

Other than the overloading issues, I couldn't think of anything else.

Edit: I've since found this C++ FQA entry (20.8) which agrees with the above - the presence of overloading creates a serious problem for parameter contra-variance.