private (name mangled) attributes within inherited classes can not have different types

Bug Report

mypy does not recognise that a dunder class attribute does not clash with the inherited one

To Reproduce

Consider the following example:

class BaseSpecialValue:

    def __init__(self, special_value:int):
        self.__special_value = special_value

    @property
    def base_special_value(self) -> int:
        return self.__special_value

class SpecialValue(BaseSpecialValue):

    def __init__(self, base_special_value:int, special_value:str):
        super().__init__(special_value=base_special_value)
        self.__special_value = special_value

    @property
    def special_value(self) -> str:
        return self.__special_value

if __name__ == '__main__':

    foo = SpecialValue(special_value='one', base_special_value=1)
    print(f'special_value={foo.special_value}, base_special_value={foo.base_special_value}')

In this case both SpecialValue and BaseSpecialValue define __special_value, when run it is clear that python treats them separately as the result is:

special_value=one, base_special_value=1

Expected Behavior

No mypy error reported

Actual Behavior

However, when mypy is run the following result is returned by mypy:

mypy --config-file=default_mypy.ini mypy_dunder_test.py   
mypy_dunder_test.py:15: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
mypy_dunder_test.py:19: error: Incompatible return value type (got "int", expected "str")  [return-value]
Found 2 errors in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.17.1
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): See below
• Python version used: 3.12.0

default configuration used

# Global options:

[mypy]
warn_return_any = True
warn_unused_configs = True

Beginne damit, das bereitgestellte Vererbungsbeispiel mit mypy 1.17.1 auszuführen und die beiden gemeldeten Fehler zu bestätigen. Verfolge, wie mypy Namen von Attributen mit doppeltem Unterstrich über geerbte Klassen hinweg behandelt, und überprüfe anschließend, dass das Beispiel keine Fehler erzeugt, während die unterschiedlichen inferierten Typen und Rückgabetypen der Properties erhalten bleiben.