Type narrowing with TypeGuards on unions does not work for else branches

[timhoffm]

Bug Report

Example

from typing import TypeGuard, reveal_type


def is_int(obj: int | str) -> TypeGuard[int]:
    return isinstance(obj, int)


def whats_this(obj: int | str) -> None:
    if is_int(obj):
        reveal_type(obj)
    else:
        reveal_type(obj)


whats_this(1)       # Revealed type is "builtins.int"
whats_this("test")  # Revealed type is "Union[builtins.int, builtins.str]"

Expected Behavior

whats_this("test") should reveal "builtins.str".

Note: This already works correctly for type narrowing expressions, i.e. when changing if is_int(obj) to if isinstance(obj, int).

Actual Behavior

The type in the else branch is not narrowed.

Your Environment

• Mypy version used: 1.15.0
• Python version used: 3.12

[JelleZijlstra]

This is correct behavior; TypeGuard does not narrow in the else branch. See e.g. https://typing.python.org/en/latest/guides/type_narrowing.html#typeis-and-typeguard .