Field of union needs to be different type when reading vs setting

Bug Report

Mypy appears to infer that the type of a field of a variable that is typed as a union is a single type whether that field is being read or updated. The type correct when reading the field (union of the types of the fields) , but not correct when setting as it does not enforce that the value being assigned needs to be valid for both of the types in the union (intersection of the types of the fields).

To Reproduce

class Foo:
    a: int

class Bar:
    a: str

either: Foo | Bar
either.a = 1  # No error, despite not being valid for Bar

More detailed example: https://mypy-play.net/?mypy=latest&python=3.12&gist=bf74f1ba35979f10b1878b2e1abca865

Expected Behavior

Mypy should report the assignment as an error if it does not work with all the types in the union.

Actual Behavior

No error, despite the assignment allowing a field of type str to be an int

Commencez par exécuter l’exemple Foo/Bar fourni et la reproduction mypy-play liée afin de comparer les lectures de champs aux affectations sur une union. Suivez la vérification pertinente des affectations d’attributs d’une union, puis confirmez que le problème est résolu lorsque les affectations ne sont acceptées que si elles sont valides pour chaque membre de l’union et que l’exemple signalé produit une erreur.