"Access to generic class variables is ambiguous" with CRTP

[nuiva]

Bug Report

Mypy complains that "Access to generic class variables is ambiguous" when using the CRTP technique, even though the type of the class variable in question is actually known.

To Reproduce

Save the following Python program:

from typing import TypeVar, Type, Generic, ClassVar, Any

T = TypeVar("T", bound="Base[Any]")

class Base(Generic[T]):
    instance_list: ClassVar[list[T]]
    @classmethod
    def init(cls: Type[T]) -> None:
        cls.instance_list = []

class Derived(Base["Derived"]):
    pass

Derived.init()
print(Derived.instance_list)

Actual Behavior

Executing mypy test.py outputs:

test.py:15: error: Access to generic class variables is ambiguous
Found 1 error in 1 file (checked 1 source file)

Expected Behavior

Mypy should not output errors, because Derived inherits the class Base[Derived] and therefore Derived has the member variable instance_list: ClassVar[list[Derived]]. Clearly the type is not generic or ambiguous at this point.

Your Environment

• Mypy version used: mypy 0.910
• Mypy command-line flags: none, see above
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: Python 3.9.6
• Operating system and version: Windows 10.0.19042

[brianschubert]

The current behavior is intentional - mypy doesn't support type variables in ClassVar annotations, which is consistent with the typing spec and PEP 526. There have been some discussions about allowing this in some form (e.g. python/typing#1424), but so far no concrete proposal has been made (to my knowledge). If there's interest in this sort of change, someone would need to start the process to amend the typing spec by following the steps outlined here.

Since the current behavior is consistent with the typing spec, I'm going to close this issue for now.