Instance attributes specified in protocols should not be usable as class attributes in inheriting concrete classes

Are you reporting a bug, or opening a feature request?

Bug

Minimal repro

from typing import Protocol

class SomeProtocol(Protocol):
    y : int
    x : int = 3

class Class(SomeProtocol):
    pass

def test1() -> None:
    y: int = Class.y  # runtime error. mypy should catch it?

def test2() -> None:
    x: int = Class.x  # runs fine, but should probably be a mypy error?

What is the actual behavior/output?

mypy reports no errors.

What is the behavior/output you expect?

I would expect mypy to complain that Class does not have class attributes x nor y (they are not marked with ClassVar).

What are the versions of mypy and Python you are using?

mypy: 0.782
python: 3.8.3

What are the mypy flags you are using? (For example --strict-optional)

No flags

Inizia tracciando come mypy gestisce l’accesso agli attributi nelle sottoclassi concrete di Protocol; l’issue non indica alcun file sorgente né alcun test esistente. Aggiungi un test di regressione basato sull’esempio SomeProtocol/Class fornito e considera il lavoro completato quando gli accessi sia a x sia a y vengono rifiutati perché privi di ClassVar.