iter doesn't work on TypeVar with bound of Enum

[bmerry]

Bug Report

A function that takes an argument of Type[_E] where _E = TypeVar("_E", bound=Enum) does not accept calling iter on that argument. Calling iter on a concrete enum class is accepted, which is why I think this isn't purely a typeshed issue. Using the class in a loop or calling the __iter__ member is also accepted.

To Reproduce

Run mypy on the following:

#!/usr/bin/env python3

import socket
from enum import Enum
from typing import Iterator, Type, TypeVar

_E = TypeVar("_E", bound=Enum)

def enum_iter(cls: Type[_E]):
    return iter(cls)

for value in enum_iter(socket.SocketKind):
    print(value)

Expected Behavior

Expected no mypy errors.

Actual Behavior

mypy_enum_iter.py:10: error: No overload variant of "iter" matches argument type "Type[_E]"
mypy_enum_iter.py:10: note: Possible overload variants:
mypy_enum_iter.py:10: note:     def [_SupportsNextT <: SupportsNext[Any]] iter(_SupportsIter[_SupportsNextT]) -> _SupportsNextT
mypy_enum_iter.py:10: note:     def [_T] iter(Callable[[], Optional[_T]], None) -> Iterator[_T]
mypy_enum_iter.py:10: note:     def [_T] iter(Callable[[], _T], object) -> Iterator[_T]
Found 1 error in 1 file (checked 1 source file)

What's even weirder is that adding an Iterator[_E] return annotation to enum_iter makes it pass.

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.8.10
• Operating system and version: Ubuntu 20.04

[AlexWaygood]

I think this is a repro that doesn't involve enums, revealing that the issue is a general one with mypy's understanding of metaclasses rather than an issue specific to the enum plugin:

from typing import Iterator, TypeVar

M = TypeVar("M")

class Meta(type):
    def __iter__(self: type[M]) -> Iterator[M]: ...

class Foo(metaclass=Meta): ...

F = TypeVar("F", bound=Foo)

def foo_iter(cls: type[F]):
    return iter(cls)

Mypy output:

main.py:13: error: No overload variant of "iter" matches argument type "Type[F]"
main.py:13: note: Possible overload variants:
main.py:13: note:     def [_SupportsNextT <: SupportsNext[Any]] iter(__iterable, _SupportsIter[_SupportsNextT]) -> _SupportsNextT
main.py:13: note:     def [_T] iter(__function, Callable[[], Optional[_T]], None) -> Iterator[_T]
main.py:13: note:     def [_T] iter(__function, Callable[[], _T], object) -> Iterator[_T]
Found 2 errors in 1 file (checked 1 source file)

[AlexWaygood]

It probably doesn't even have anything to do with TypeVars. I can reproduce a similar error with this snippet:

from typing import Iterator, TypeVar

M = TypeVar("M")

class Meta(type):
    def __iter__(self: type[M]) -> Iterator[M]: ...

class Foo(metaclass=Meta): ...

def foo_iter(cls: type[Foo]):
    return iter(cls)  # error: Argument 1 to "iter" has incompatible type "Type[Foo]"; expected "_SupportsIter[<nothing>]"

[bmerry]

I guess a good question is whether subclasses of Foo are expected to have a metaclass that is a subclass (in the Liskov substitution sense) of Meta. If not, then the error would be legitimate since cls could be an instance of a subclass of Foo whose metaclass doesn't provide __iter__.