Specialize empty collections to Any as a fallback

Context

I have a pending PR for typeshed which fixes overparameterized dict.get and dict.pop methods: python/typeshed#10294. The PR causes a lot of failures in various GitHub projects due to how mypy treats empty collections. Specifically, if dict.get is annotated as

class dict(Generic[KT, VT]):
    def get(self, key: KT, default: T) -> T | VT: ...

mypy infers the type of v in

def test(d: dict[str, Any]):
    v = d.get("foo", {})

to be Any | dict[<unknown>, <unknown>], instead of Any | dict[Any, Any].

Feature

Specialize empty collections to Any if no other information is available to recover type arguments.

Pitch

Using <unknown> instead of Any requires users to cast or add type annotations where they otherwise will not be necessary. See comments in the PR for a few examples from real projects.

Commencez par reproduire l’exemple montré de dict.get dans mypy et examinez comment les expressions de collections vides sont inférées lorsqu’aucun argument de type ne peut être récupéré. Le travail est terminé lorsque les collections vides dans cette situation sont spécialisées en Any plutôt qu’en unknown, et que le comportement reste couvert par des tests pertinents de vérification des types.