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

Inizia eseguendo l’esempio Foo/Bar fornito e la riproduzione collegata di mypy-play per confrontare le letture dei campi con le assegnazioni su una union. Traccia il controllo pertinente delle assegnazioni degli attributi di una union, quindi conferma che il problema è risolto quando le assegnazioni vengono accettate solo se sono valide per ogni membro della union e l’esempio segnalato produce un errore.