False positive when using mypy and assuming PEP 724 is covered

[SMoraisAnsys]

Hey guys, first of all thanks for this wonderful tool !

My report might not be considered a bug if you consider that PEP 724 isn't handled by mypy. If that's the case, let me know and I can open a new ticket dedicated to adding PEP 724. This being said, I assumed that PEP 724 was covered by mypy but it doesn't seem to be the case. Here is a dummy reproducer

from dataclasses import dataclass
from typing import TypeGuard, Union

@dataclass
class ThermalModel:
    conductivity: float

@dataclass
class MechanicalModel:
    stiffness: float

Model = Union[ThermalModel, MechanicalModel]

def is_valid_thermal_model(obj: Model) -> TypeGuard[ThermalModel]:
    return hasattr(obj, "conductivity") and obj.conductivity >= 0

def run_simulation(model: Model):
    if is_valid_thermal_model(model):
        model.conductivity *= 2.0
    else:
        model.stiffness *= 1.1

Since is_valid_thermal_model is False, I would expect the type to be narrowed to MechanicalModel when working in the else statement. However, I get the following error error: Item "ThermalModel" of "ThermalModel | MechanicalModel" has no attribute "stiffness" [union-attr]. My understanding here is that PEP 647 is covered but not PEP 724.

Note that if I apply another if statement with TypeGuard instead of the else statement, it works fine. Here would be the changes

def is_valid_mechanical_model(obj: Model) -> TypeGuard[MechanicalModel]:
    return hasattr(obj, "stiffness") and obj.stiffness >= 0

def run_simulation(model: Model):
    if is_valid_thermal_model(model):
        model.conductivity *= 2.0
    elif is_valid_mechanical_model(model):
        model.stiffness *= 1.1

Expected Behavior

I wouldn't expect any error with the previous snippet.

Actual Behavior

The error obtained is error: Item "ThermalModel" of "ThermalModel | MechanicalModel" has no attribute "stiffness" [union-attr].

Your Environment

• Mypy version used: mypy 1.18.2 (compiled: yes)
• Mypy command-line flags: mypy myfile_.py
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10

Note that the version used is the latest version available on PyPI.