Adding third ABC type to a loop causes false positive type error

[johnthagen]

Bug Report

Adding a third ABC to a list of types in a for loop causes a Mypy type checking false positive error.

In the code below, if you remove the second C2 there is no error, but adding a third ABC to the iteration for some reason triggers this error.

To Reproduce

• https://mypy-play.net/?mypy=latest&python=3.10&gist=273ca66221a89cf15a8ff1c521a067e3

from abc import ABC, abstractmethod


class Base(ABC):
    @abstractmethod
    def a(self) -> None:
        ...


class C1(Base):
    def a(self) -> None:
        return


class C2(Base):
    def a(self) -> None:
        return


for c in (C1, C2, C2):
    c()

Expected Behavior

No type check error.

Actual Behavior

Produces false positive type check error:

main.py:24: error: Cannot instantiate abstract class "Abstract" with abstract attribute "a"  [abstract]

Your Environment

• Mypy version used: 1.7.0
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files):

[tool.mypy]
strict = true

• Python version used: 3.10.11

[AlexWaygood]

I think this is just another instance of #3115?

[JelleZijlstra]

It's true though that mypy behaves differently depending on whether you iterate over (C1, C2) or (C1, C2, C2). Here's a slightly expanded mypy play: https://mypy-play.net/?mypy=latest&python=3.10&gist=41c9f315bc22c5c19c40f60196042567

I'm not sure how to explain that change. reveal_type() doesn't change between the two cases.

[KotlinIsland]

If we change it to basedmypy we can see that it's working as expected
https://mypy-play.net/?mypy=basedmypy-latest&python=3.10&gist=41c9f315bc22c5c19c40f60196042567

[hauntsaninja]

Fixed in #17408