mypy silent about incorrect metaclasses-based protocol

[elazarg]

from typing import TypeVar, Type, Iterable, Iterator
T = TypeVar('T')
class M(type):
    def __iter__(self: Type[T]) -> Iterator[T]: raise Exception
class A(metaclass=M): pass
x: Iterable[A] = A  # fine
y: Iterable[str] = A  # should be rejected, but it is not

This pattern is important for enums, and came up in python/typeshed#1755. @ilevkivskyi noted that this is likely due to a bug in bind_self (see there).

[ilevkivskyi]

Thanks for opening the issue! It looks like the fix is more complex, since it requires carrying over original_type from subtypes.py.

[elazarg]

In general bind_self is buggy and ad-hoc. But when I tried to do it "right" it feels like it means making a class MemberChecker with all the context carried around by ExpressionChecker and TypeChecker.