--warn-unreachable and isinstance leads to false positive about method resolution order

Bug Report

A small V shaped class hierarchy, a variable annotated as an instance of the supertype, and a few assert isinstance to progressively narrow the type gets a false positive error message from mypy that no such subclass could exist. The code works fine at runtime, though.

To Reproduce

https://gist.github.com/mypy-play/a653520575a1e3279bfcec2f64c4a128

Requires --warn-unreachable:
https://mypy-play.net/?mypy=latest&python=3.11&flags=warn-unreachable&gist=a653520575a1e3279bfcec2f64c4a128

class A: pass
class B(A): pass
class C:pass
class D(B, C):pass

m: A = D()
assert isinstance(m, C)  # remove this line, or move it to after the following one, and the error goes away.
assert isinstance(m, D)  # error: Subclass of "A", "C", and "D" cannot exist: would have inconsistent method resolution order  [unreachable]

Actual Behavior

$ mypy --warn-unreachable b.py
b.py:8: error: Subclass of "A", "C", and "D" cannot exist: would have inconsistent method resolution order  [unreachable]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.2.0 (compiled: yes)
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.0rc1

Start with the reproducer in the issue and run mypy with --warn-unreachable to confirm the diagnostic. Trace the isinstance narrowing and method-resolution-order analysis responsible for the unreachable result; done means this valid hierarchy no longer produces a false positive while genuine unreachable cases remain reported.