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

Neither, however, support contra-variance in parameter types - instead, it translates to overloading (Java) or hiding (C++).

Why is that? It seems to me that there is no harm in allowing that. I can find one reason for it in Java - since it has the "choose-the-most-specific-version" mechanism for overloading anyway - but can't think of any reason for C++.

Example (Java):

class A {
    public void f(String s) {...}
}
class B extends A {
    public void f(Object o) {...} // Why doesn't this override A.f?
}