Field as a part of a union seems to be broken

[budziam]

Bug report

What's wrong

Django model Field value as a part of a union results in unexpected behavior. Please find the following example:

from typing import Union
from uuid import UUID

from django.db import models


class User(models.Model):
    uuid = models.UUIDField()


class AnonymousUser:
    uuid: UUID


def example(value: UUID) -> None:
    ...


user_only: User = User()
anonymous_only: AnonymousUser = AnonymousUser()
union_value: Union[AnonymousUser, User] = User()

example(user_only.uuid)
example(anonymous_only.uuid)
example(union_value.uuid)

It causes the following error

test.py:25: error: Argument 1 to "example" has incompatible type "Union[UUID, UUIDField[Union[str, UUID], UUID]]"; expected "UUID"

How is that should be

No error should be raised.

System information

• OS: macos
• python version: 3.8.9
• django version: 3.2.6
• mypy version: 0.910
• django-stubs version: 1.8.0

[flaeppe]

This is an upstream problem for classes with descriptors, ref: python/mypy#5570 (comment)

I'm gonna close this in favour of the mypy issue.