Bad error message while checking subclass of generic classes

Bug Report

mypy command on a misspecified subclass of a generic class returns a pretty confusing error message (As per issue #8696)

To Reproduce

1. Create a script containing this snippet:

from abc import ABC, abstractmethod
from typing import TypeVar, Iterable, Iterator

T = TypeVar('T')


class MyABC(ABC):

    @property
    @abstractmethod
    def items(self) -> Iterable[T]: ...

    def my_method(self) -> Iterator[T]:
        for item in self.items:
            yield item

2. Run mypy on the created script

Expected Behavior

Since MyABC should inherit also from Generic[T] I'd expect that the error message would return something on the line of:

error: Incompatible types in "yield". The class should inherit from "Generic[T]"

Actual Behavior

The actual error message is:

error: Incompatible types in "yield" (actual type "T", expected type "T")

Environment

• Mypy version used: 0.910
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.6.5
• Operating system and version: MacOS BigSur 11.5.2

Inizia eseguendo mypy sullo script minimale nell’issue e conferma la diagnostica confusa di yield. Non viene indicato alcun file del repository né alcun test; traccia la diagnostica dal punto di ingresso del comando mypy, quindi verifica che la sottoclasse generica specificata in modo errato produca un messaggio utilizzabile che identifichi l’ereditarietà mancante da Generic[T].