TypeGuard does not narrow when assignment expression

[roganov]

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

[AlexWaygood]

This is really weird. Here's a slightly more minimal repro:

import typing

s: str | int


def is_str(x: str | int) -> typing.TypeGuard[str]:
    return y if (y := False) else False


if is_str(s):
    s + ''

https://mypy-play.net/?mypy=latest&python=3.12&gist=592d718662024c2496121bd58e85f5fd

[JelleZijlstra]

This also reproduces it:

import typing

s: str | int


def is_str(x: str | int) -> typing.TypeGuard[str]:
    return y


if is_str(s):
    s + ''

Or

def is_str(x: str | int) -> typing.TypeGuard[str]:
    return True + "x"

It (correctly) emits an error inside the TypeGuard, but then also disables the TypeGuard.

[AlexWaygood]

So there's two bugs here:

1. Mypy gets confused about a typeguard function that returns a boolean conditions if the boolean condition involves a walrus expression. It thinks that it's an invalid typeguard function when it should see it as valid.
2. If mypy sees something it deems to be an invalid typeguard function, the error "leaks" out to places where the typeguard function is called, when mypy should just emit an error at the location where the typeguard function is originally defined.

[jaycle]

I just opened #17104, but looking at comments, they may be related.