Add support for classmethod property from python3.9

[YaN-3k]

class Foo:
    @classmethod
    @property
    def bar(cls) -> str:
        return cls.__name__ 


def foo_bar(msg: str) -> None:
    print(msg)

foo_bar(Foo.bar)

mypy --python-version 3.9 test.py
test.py:11: error: Argument 1 to "foo_bar" has incompatible type "Callable[[], str]"; expected "str"
Found 1 error in 1 file (checked 1 source file)

As stated in https://docs.python.org/3.9/library/functions.html#classmethod this new classmethod property is completely valid so mypy shouldn't report an error.

[AlexWaygood]

It might be worth noting that there are currently active discussions on whether to deprecate the ability to stack @classmethod on top of @property, as it's responsible for a lot of bugs in cpython

[AlexWaygood]

It might be worth noting that there are currently active discussions on whether to deprecate the ability to stack @classmethod on top of @property, as it's responsible for a lot of bugs in cpython

To update: this behaviour has now been deprecated in 3.11 (python/cpython#92379)

[headtr1ck]

This has been deprecated in python 3.11 it is still correct syntax in python <3.11.
However, it is still semantically incorrent and mypy is right in complaining (the property decorator is not returning what you think it returns).

Maybe one could simply improve the error message for this case?

[karlicoss]

Would it be possible to change the error category from misc to something more specific? Then it would be easy to suppress it on older python versions

[hauntsaninja]

This behaviour has been deprecated in CPython, so mypy will not be adding support for it.