If a class is inherited and a feature/method/attribute redefined, a parameter or return type can be changed to be a more specialised type.

So if we have a feature A that takes a formal argument T. We redefine A, the new A must have a formal argument of T, or any descendent of T.

Covariant parameters are not safe, but return types are. In Eiffel you can get compile errors , because the polymorphism breaks.