[loeg]

I think it is wrong/weird for objects of the same type to have different vtables, yes. I would call those different types.

Upcasting is fine, but generally speaking the expected behavior of invoking a superclass method on an object that is actually a subclass is that the subclass method implementation is used (in C++, this would be a virtual/override type method, as opposed to a static method). Invoking a superclass-specific method impl on a subclass object is kind of weird.

[1718627440]

In most languages, this is not possible, because they abstract over the implementation of classes. In C it is so you can be more creative. You can for example use it instead of a flag for behaviour. Why branch on a variable and then call separate methods, when you can simply assign the wanted implementation directly. If you want to know, which implementation/mode is used, comparing function pointers and scalar variables amounts to the same. It is also an easy way to get a unique number. When all implementations of that can operate on the same type, they are interchangeable.

In C you can also change the "class" of an instance as needed, without special syntax. Maybe you need to already call a method of the new/old class, before/after actually changing the class type.

> is that the subclass method implementation is used

The entire point of invoking the superclass method is, because the subclass has a different implementation and you want to use the superclass implementation.