Dataclass + Generic fails during sleight of hand

[rtpg]

With the following snippet:

from dataclasses import dataclass
from typing import Generic, TypeVar

T = TypeVar('T')

@dataclass
class DebugVisitor(Generic[T]):
    tree_depth: int = 0

print("DONE")

Loading the module blows up with the following trace:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/home/rtpg/proj/pycomp/dc_example.py", line 11, in <module>
    class DebugVisitor(Generic[T]):
AttributeError: attribute '__dict__' of 'type' objects is not writable

When we have the Generic + dataclass combo here, in CPy_DataclassSleightOfHand we end up trying to re-assign __dict__ to a type after applying the dataclass decorator. This issue doesn't show up if we aren't using the Generic[T] subclass.

I haven't fully isolated what's going on here, just yet though. Just wanted to at least document the issue. It kinda feels like there's a __dict__ check possible here (one thing I noticed is that in __mypyc_attrs__ we have tree_depth and __dict__ with the dataclass + Generic, but if we just have the dataclass we only have tree_depth).

[JukkaL]

That dataclass code is quite tricky. In the long term we may want to completely compile away the @dataclass decorator, since it's such a basic language feature, but for now some smaller-case fix would be good to have for this issue.