"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