`classmethod`s shouldn't be allowed to access generic attributes

from typing import Generic, TypeVar

T = TypeVar("T")


class A(Generic[T]):
    class_attr: T

    @classmethod
    def set_attr(cls, arg: T) -> None:
        cls.class_attr = arg  # no error?


B = A[str]
B.set_attr("1")
B.class_attr = "1"  # error: Access to generic instance variables via class is ambiguous

C = A[int]
C.set_attr(1)
C.class_attr = 1  # error: Access to generic instance variables via class is ambiguous

value = B().class_attr

reveal_type(value)  # revealed: str, actual value: int

Start by running the standalone Python reproduction in the issue and compare the diagnostics for classmethod access with direct class assignment. The fix is complete when generic attributes accessed through a classmethod are rejected consistently and the revealed type cannot become unsound; no source file or test path is named.