AnyStr parameter + `sys.version_info` check in function causes mypy to not realize True is bool

[jwodder]

Bug Report

Consider the following code:

from __future__ import annotations
import sys
from typing import AnyStr


def foo(s: AnyStr) -> bool:
    if sys.version_info >= (4, 42):  # Something that won't be matched on any extant Python
        return True
    return False

Running mypy --warn-return-any on this code produces the following erroneous error:

bool04.py:8: error: Returning Any from function declared to return "bool"  [no-any-return]
Found 1 error in 1 file (checked 1 source file)

If the type of the s parameter is changed to str, or the parameter is removed entirely, then the error goes away. The error also goes away if the sys.version_info check is something that succeeds on the current Python.

Expected Behavior

mypy should not have reported any errors.

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: --warn-return-any
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: observed on Python 3.8.20, Python 3.9.20, Python 3.12.7, and Python 3.13.0

[brianschubert]

Can confirm that the required ingredients are an always-false sys.version_info check (or similar, not TYPE_CHECKING also works) and a constrained TypeVar.

Even funkier:

T  = TypeVar("T", int, str)

def foo(x: T) -> bool:
    if sys.version_info >= (4, 42):
        x = "foo"
        return True  # E: Incompatible return value type (got "str", expected "bool")
    return False