There was an experimental language called Cecil that had "conditional inheritance". You could say things like class Rectangle: var w,h class S

There was an experimental language called Cecil that had "conditional inheritance". You could say things like class Rectangle: var w,h class Square(Rectangle if w == h): ... You could then overload functions/methods on both `Square` and `Rectangle` and it would call the correct overload depending on the runtime values of `w` and `h`.