Non-inline `__init_subclass__` definition not recognized, despite giving type hints.

Mypy does not recognize overwritten __init_subclass__ and complains that it's different from object's __init_subclass__.

So a simple code would be something like:

import dataclasses as dcls
from typing import Protocol, TypeVar, Callable

class InitSubclassWithKey(Protocol):
    """
    The ``__init_subclass__`` method of the classes making use of a factory.
    """

    def __call__(self, cls, *, key: str = "") -> None: ...

GLOBAL_REGISTRY = {}

def generate_init_subclass[T](lazy_base_class: Callable[[], T]) -> InitSubclassWithKey:
    def __init_subclass__(cls, *, key: str = "") -> None:
        base_class = lazy_base_class()
        GLOBAL_REGISTRY[base_class, key] = cls
        print("Registered:", base_class, key, cls)

    return __init_subclass__


class Base:
    # Have to use a lazy getter here because or else it would be ``NameError``.
    __init_subclass__ = generate_init_subclass(lambda: Base)

class Derived(Base, key="derived"):
    pass

Expected Derived to pass without issue (as is the case if I define __init_subclass__ directly).
Note: mypy has full type information here because of the protocol given (InitSubclassWithKey).

However, I get:

main.py:26: error: Unexpected keyword argument "key" for "__init_subclass__" of "object"  [call-arg]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.15.0 (compiled: yes)
• Mypy command-line flags: main.py
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.13

main.py の再現コードから始め、Python 3.13 で mypy 1.15.0 を実行して、予期しないキーワード引数エラーを確認します。mypy が init_subclass へのインラインでない代入をどのように処理し、object にフォールバックするかを追跡します。型付きの Protocol 代入によって Derived(Base, key="derived") がエラーなしで許可されれば完了です。