Bug Report

Program using protocol for attrs instances fails to typecheck once the mypy cache is warm.

To Reproduce

Given a virtualenv containing:

$ pip freeze
attrs==22.1.0
mypy==0.991
mypy-extensions==0.4.3
tomli==2.0.1
typing_extensions==4.4.0

a.py:

import attr
from typing import Protocol, ClassVar, Any

class AttrsInstance(Protocol):
    """
    A protocol to be able to statically accept an attrs class.
    attr itself defines AttrsInstance but only at type checking time.
    """

    __attrs_attrs__: ClassVar[Any]


@attr.s
class Foo:
    bar: str = attr.ib()

b.py:

from a import AttrsInstance, Foo
from typing import Type

thing: Type[AttrsInstance] = Foo 
print(thing)

First try with cold cache:

$ rm -rf .mypy_cache/  # Make sure the cache is cold
$ mypy a.py b.py
Success: no issues found in 2 source files

Now edit b.py adding one line at the end for example:

from a import AttrsInstance, Foo
from typing import Type

thing: Type[AttrsInstance] = Foo 
print(thing)

foo = False

And re-run mypy, getting error unrelated to the change!

$ mypy a.py b.py
b.py:4: error: Incompatible types in assignment (expression has type "Type[Foo]", variable has type "Type[AttrsInstance]")  [assignment]
Found 1 error in 1 file (checked 2 source files)

Deleting the cache makes it work again:

$ rm -r .mypy_cache/
$ mypy a.py b.py

Note that we also get an error if using AttrsInstance directly instead of Type[AttrsInstance]:

Using this b.py:

from a import AttrsInstance, Foo
from typing import Type

thing: AttrsInstance = Foo("bar")
print(thing)

Results in this after a similar edit to b.py:

(virtualenv) thomas@thomas-xps13:~/Projects/test$ mypy a.py b.py
b.py:4: error: Incompatible types in assignment (expression has type "Foo", variable has type "AttrsInstance")  [assignment]
b.py:4: note: Protocol member AttrsInstance.__attrs_attrs__ expected class variable, got instance variable
b.py:4: note: Protocol member AttrsInstance.__attrs_attrs__ expected settable variable, got read-only attribute
Found 1 error in 1 file (checked 2 source files)

Expected Behavior

Program should still typecheck correctly with warm cache.

Actual Behavior

Program fails to typecheck. Deleting the cache and re-running works as a (slow) workaround.

Your Environment

• Mypy version used: 0.991
• Mypy command-line flags: mypy a.py b.py
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.6 (ubuntu 22.04, package version 3.10.6-1~22.04.1)