Well if you have a Square type that's a subtype of Rectangle then you can't have methods on Rectangle that, when called on a Square instance, would invalidate t

Well if you have a Square type that's a subtype of Rectangle then you can't have methods on Rectangle that, when called on a Square instance, would invalidate the invariant of a Square. If your system is sufficiently dynamic (prototype-based?) you can exchange the class of an instance at runtime. If not, simply make your Rectangles immutable, so a setWith method returns a new instance and you can then return a new Rectangle or a new Square from a Rectangle's setWidth. That should work in any statically typed language with inclusion polymorphism.