I think this is because the annotate function for the class holds on to the original class namespace, and that namespace dict contains the descriptors B.__dict__ and B.__weakref__ that refer back to the original class. Not too sure how to fix that yet.

This seems hard to avoid, maybe we should just live with the change? The reason is that the annotation scope needs to hold a reference to the actual class dict (for reasons explained in https://jellezijlstra.github.io/pep695#class-scopes) and the class dict contains two descriptors, for __dict__ and __weakref__, that contain references back to the class object.

In Python 3.12 and 3.13 this behavior is already observable if you use a type alias or TypeVar bound in the dataclass, with one of these class definitions:

@dataclass(slots=True)
class B:
    def f[T: int](self, x: T) -> T: ...
    b: int

@dataclass(slots=True)
class B:
    type T = int
    b: int

Both of these fail the assertion in the post above.

I put up a change in #135230 that fixes this issue, but messing with the descriptors feels like it's probably going to break something else.