Using subclass type parameters in virtual functions

Question

I have this piece of code (contrived from my real-life trouble)

It cannot compile, complaining ExtendsB does not implement B::Run(A* a). However, it has no

Answer 1

Specialising the return type makes the subclass more strict - it will act as an A. However, restricting the Run method to accept only a subclass of the original argument makes B not act as an A.

Inheritance models an is-a relationship. Your code violates the Liskov Substitution principle.