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

Start by running the provided inheritance example with mypy 1.17.1 and confirming the two reported errors. Trace how mypy handles double-underscore attribute names across inherited classes, then verify that the example produces no errors while preserving the distinct inferred types and property return types.