@Raymond yeah I've been thinking about this some more, and there's no way to have a "top level" method with the dataclass decorator.

I think I will make a case to have a class version of dataclasses that works with inheritance.  Class versions of dataclasses are used in some places like here: https://github.com/google/flax/blob/main/flax/struct.py#L184
They just call dataclass on the class in __init_subclass__.

If we had something like this in the standard library, then you could put your empty __post_init__ in that class.  You could also make __init__ call super so that mixins would be initialized (right now the collider pattern you showed also breaks if B is not a dataclass, and has a non-trivial __init__).