Allow iterable class objects to be unpacked

[AlexWaygood]

Currently, mypy issues a false positive on this code, where Foo is iterable by virtue of having Meta as its metaclass:

from typing import Iterator
class Meta(type):
    def __iter__(cls) -> Iterator[int]:
        yield from [1, 2, 3]

class Foo(metaclass=Meta): ...

a, b, c = Foo  # error: "Type[Foo]" object is not iterable  [misc]
reveal_type(a)  # error: Cannot determine type of "a"  [has-type]  # note: Revealed type is "Any"

This PR fixes that false positive.

I started working on this PR hoping to address #14782, but wasn't able to -- this PR improves the situation somewhat, but doesn't close the issue. Prior to this PR, mypy had this behaviour:

from enum import Enum

class E(Enum):
    A = 1
    B = 2

A, B = E  #  error: "Type[E]" object is not iterable  [misc]

With this PR, mypy has this behaviour instead:

from enum import Enum

class E(Enum):
    A = 1
    B = 2

A, B = E  # error: Need type annotation for "A"  [var-annotated]  # error: Need type annotation for "B"  [var-annotated]

This is because there's a TypeVar being used in typeshed's stub for EnumMeta.__iter__: I wasn't able to figure out how to get mypy to solve the type variable in this context.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

I found a much better solution, which also fixes the enum issue

[AlexWaygood]

I found a much better solution, which also fixes the enum issue

• Allow iterable class objects to be unpacked (including enums) #14827