Specialize empty collections to Any as a fallback #15433

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.