Bug Report

mypy reports an error because of a concern about potentially incompatible subclasses, even though the class in question is marked @final.

To Reproduce

https://mypy-play.net/?mypy=latest&python=3.13&gist=c475ec59941e76a62c4d458819ebc828

from __future__ import annotations
from typing import Self, final

class A:
    pass

@final
class B(A):
    @final
    def __init__(self) -> None:
        pass

    @classmethod
    def __new__(cls: type[B]) -> B:
        """ cls is type[B] and return is B, because class is final """
        new_instance: B = super().__new__(cls)
        new_instance.__init__()
        return new_instance

@final
class B2(A):
    @final
    def __init__(self) -> None:
        pass

    @classmethod
    def __new__(cls) -> Self:
        new_instance: Self = super().__new__(cls)
        new_instance.__init__()
        return new_instance

Expected Behavior

No errors reported.

Note that I think marking __init__ as @final is redundant, but I included it just to show that it doesn't make a difference. I would expect no errors if the class is marked @final OR if __init__ is marked @final. Or both, of course.

I also think the first case, where I explicitly annotate the B types, should not be necessary. Because the class is @final, I would expect the second case using Self to be fine. Again, I included that just to show that the error is reported either way.

Actual Behavior

main.py:17: error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass  [misc]
main.py:29: error: Accessing "__init__" on an instance is unsound, since instance.__init__ could be from an incompatible subclass  [misc]

Your Environment

• Mypy version used: 1.15.0
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13