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

まず、提供された再現例を mypy 0.931、Python 3.10、--warn-unreachable で実行し、次にタプルに Any が含まれる場合に関係する isinstance の絞り込みロジックを追跡します。完了条件は、例が両方の分岐で m を Any と報告し、else 分岐を到達不能としてマークしなくなることです。