TypeGuard does not narrow when assignment expression

Bug Report

In the example below, is_str is a type guard, but when used mypy fails to narrow the type. I'm pretty sure this happens because of the assignment expression.

To Reproduce
https://mypy-play.net/?mypy=latest&python=3.12&gist=42da00a72ace0a51a5d2335721877591

import typing

s: str | int


def is_str(x: str | int) -> typing.TypeGuard[str]:
    return bool(
        isinstance(x, str)
        and (y if len(y := '123'.split('/')) > 1 else '')
    )


if is_str(s):
    s + ''

Expected Behavior
No mypy errors.

Actual Behavior

main.py:14: error: Unsupported operand types for + ("int" and "str") [operator]
main.py:14: note: Left operand is of type "str | int"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: 1.19
• Mypy command-line flags: non
• Mypy configuration options from mypy.ini (and other config files): non
• Python version used: 3.12

Start with the mypy-play reproducer and inspect how the TypeGuard call is analyzed when the guard contains an assignment expression. Use main.py's reported line as the expected narrowing check; done means the unsupported operand error is gone and the existing reproduction passes.