It already does too. Just remove the `type` bit and you have an alias for a union, which has been supported since typing was added (though the `|` syntax was on

It already does too. Just remove the `type` bit and you have an alias for a union, which has been supported since typing was added (though the `|` syntax was only added in 3.10). And AFAIK the type checkers do handle completeness on unions.