Keyword arguments to `__str__` are rejected

class X:
    def __str__(self, arg: int | None = None) -> str:
        return f"{arg}"

def user(x: X) -> str:
    return x.__str__(arg=42)

main.py:2: note: "__str__" of "X" defined here
main.py:6: error: Unexpected keyword argument "arg" for "__str__" of "X"  [call-arg]

https://mypy-play.net/?mypy=latest&python=3.10&gist=b0426a5fffb000335d04492959d69b34

This works fine at runtime. Interestingly, mypy allows passing additional positional arguments to __str__, e.g. x.__str__(42). I assume something is overzealously applying the general rule that dunders usually don't accept keyword arguments.

Start by reproducing the example in the linked mypy playground and inspect the type-checking path for calls to str. Compare keyword-argument handling with the already accepted positional-argument case; done means the keyword call is accepted without weakening unrelated dunder-call checks.