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.

まず、示されている dict.get の例を mypy で再現し、型引数を復元できない場合に空のコレクション式がどのように推論されるかを調べます。この状況で空のコレクションが unknown ではなく Any に特殊化され、その動作が関連する型チェックテストで引き続きカバーされていれば完了です。