Incorrect type narrowing for select class hierarchies

[Redoubts]

Bug Report

I've found that in an exhaustive isinstance if block, mypy still leaves a more general type resolution than it should. In the snippet of code, mypy should be able to understand that there is no attr-defined error.

To Reproduce

from __future__ import annotations


class A:
 pass


class B(A):
 def get_x(self):
 pass


class C(A):
 def get_x(self):
 pass


class D(C):
 pass


class E(C):
 pass


class F(C):
 pass


def x(obj: A):
 value = ""
 if isinstance(obj, B):
 value = obj.__class__.__name__
 elif isinstance(obj, D):
 value = obj.__class__.__name__
 elif isinstance(obj, E):
 value = obj.__class__.__name__
 elif isinstance(obj, F):
 value = obj.__class__.__name__
 else:
 raise RuntimeError("unhandled type")

 reveal_type(obj)
 obj.get_x()

Expected Behavior

x.py:43: note: Revealed type is "B | D | E | F"

Actual Behavior

x.py:43: note: Revealed type is "x.A"
x.py:44: error: "A" has no attribute "get_x" [attr-defined]

Your Environment

• Mypy version used: mypy 1.6.1 (compiled: yes)
• Mypy command-line flags: default
• Mypy configuration options from mypy.ini (and other config files): default
• Python version used: 3.10.13

[erictraut]

This is because mypy uses a join operator rather than unions when combining types from different code paths. join(B, D, E, F) is A. If it used unions instead, the resulting type of obj would be B | D | E | F, and no type error would result.

[hauntsaninja]

Fixed in #18538