`isinstance` removes `cls.__dict__["__annotations__"]` in a complex setup with Protocol and ABCMeta

Bug report

Bug description:

import abc
from collections.abc import Iterator
from typing import Protocol

# ---- Expected behavior:
class Steppable(metaclass=abc.ABCMeta):
  pass

class Module:
  @classmethod
  def __init_subclass__(cls, **kwargs) -> None:
    super().__init_subclass__(**kwargs)
    name, annotation, default = "name", str | None, "abc"
    cls.__annotations__[name] = annotation
    setattr(cls, name, default)


class SequenceLayer1(Module, Steppable):
  pass


print(SequenceLayer1.__dict__.get("__annotations__"))
# {'name': str | None}
assert SequenceLayer1.__dict__.get("__annotations__") is not None


# ---- Unexpected behavior:
class CheckpointableIterator(Iterator, Protocol):
  pass

isinstance(CheckpointableIterator, Iterator)

class SequenceLayer2(Module, Steppable):
  pass

print(SequenceLayer2.__dict__.get("__annotations__"))
# None
assert SequenceLayer2.__dict__.get("__annotations__") is not None

• Failing on 3.12 and 3.13
• Passing on 3.11 and 3.14 (For 3.14 it is expected to have __dict__["__annotations__"] missing)

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Python 3.12 と 3.13 で Module.init_subclass、CheckpointableIterator、Protocol、ABCMeta、isinstance を含む再現コードを実行し、その後 3.11 および 3.14 と比較します。SequenceLayer1 と SequenceLayer2 が annotations をどのように受け取るかを追跡し、2 番目のクラスがどの箇所でエントリを失うのかを特定します。動作が説明され、影響を受けるバージョン範囲をカバーする回帰テストがあれば完了です。