isinstance wrongly narrows away Any

Bug Report

Mypy falsely narrows a type when using isinstance with a tuple containing a variable of type Any.
This only happens when there is at least one other element in the tuple.
Note that a class might have type Any if it is imported from an untyped module.

To Reproduce

from typing import Any

class A: 
    pass

B: Any  # (Or from untyped_lib import B)

m: Any

if isinstance(m, (A, B)):
    reveal_type(m)
else:
    reveal_type(m)

Expected Behavior

Mypy reports no error. m is of type Any in both branches

Actual Behavior

The second branch is reported as unreachable.

Your Environment

• Mypy version used: 0.931
• Mypy command-line flags: --warn-unreachable
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10

Comienza ejecutando la reproducción proporcionada con mypy 0.931, Python 3.10 y --warn-unreachable; después, sigue la lógica de narrowing de isinstance implicada cuando una tupla contiene Any. Se considera terminado cuando el ejemplo informa de que m es Any en ambas ramas y ya no marca la rama else como inalcanzable.