Regression: false positive avoid redundant cast with literal union

This example started generating an error after #18588, which looks like a regression:

# mypy: warn-redundant-casts

from typing import Literal, cast

Foo = Literal["a", "b"]

class C:
    def __init__(self) -> None:
        self.x = cast(Foo, "a")  # error: Redundant cast to "Literal['a', 'b']"

I haven't looked into this in detail, but it seems possible that we infer a union type for "a" in a literal union context, which doesn't look right. If this is a correct hypothesis, #18588 wouldn't actually be the root cause, and it would just be exposing a pre-existing issue.

cc @asottile as the author of the PR

Inizia riproducendo l’esempio dell’issue #19055 con l’impostazione warn-redundant-casts di mypy, quindi esamina il comportamento introdotto da #18588 e il percorso di inferenza di literal-union. Il lavoro è completato quando il cast nell’esempio non viene più segnalato come ridondante e un test di regressione copre questo caso.