__class_getitem__ is not assumed to be a classmethod

[keysmashes]

mypy (version 0.720) gives confusing errors like this:

error: The erased type of self "Type[_Type]" is not a supertype of its class "_Type"

for the following method:

_TypeT = TypeVar("_TypeT", bound="_Type")
class _Type:
    def __class_getitem__(cls: Type[_TypeT], item: Any) -> _TypeT:
        raise NotImplementedError

even though this is apparently the correct way to type-hint factory functions.

Adding the @classmethod decorator makes the error go away, but according to PEP 560, __class_getitem__ is automatically a class method, so the decorator shouln't be required.

[ilevkivskyi]

Yes, we should recognise it as a class method automatically, this is however quite low priority IMO, because it is a rarely used method.

[ilevkivskyi]

On the other hand it should be easy to fix, so please go ahead if you would like to make a PR.

[keysmashes]

I think we have different definitions of "easy" :)

I pushed a few commits to my fork, but I get some errors in the tests I added, which I have no idea how to fix, since I basically just copied bits of what is special-cased for __init_subclass__ (which is also automatically a classmethod), ignoring the parts about setting the return type to None; I probably don't have the brainspace to work out how to fix this in the near future, sorry.

[ilevkivskyi]

I think we have different definitions of "easy" :)

I have only one scale, it is much easier than any other mypy issue.

Btw a quick fix for your issue will be to tweak shared_parse.special_function_elide_names() so that it will preserve cls argument name for __class_getitem__. Or maybe a better solution would be to always keep the name of first argument.

[tavianator]

I think something similar to #7355 would work for this.