[red-knot] add support for typing.cast

[carljm]

This should be pretty simple to add, once support for assert_type (and functions that take some value expressions and some type expressions) lands.

[InSyncWithFoo]

I'm working on this. It turned out not to be so simple, due to how bind_call() handles cast()'s signature:

cast(str, True)

let Some((casted_ty, _)) = binding.two_parameter_tys() else { /* ... */ };
//        ^^^^^^^^^ str | Literal[True]

A possible reason is that cast() is defined using overloads and bind_call() can't handle overloads yet:

@overload
def cast(typ: type[_T], val: Any) -> _T: ...
@overload
def cast(typ: str, val: Any) -> Any: ...
@overload
def cast(typ: object, val: Any) -> Any: ...

[InSyncWithFoo]

Open question: Should there be a "strict casting" mode in which a value cannot be casted to a type with which its own inferred type does not overlap (or of which it is not a supertype/subtype)?

Basedpyright calls this reportInvalidCast.

[carljm]

turned out not to be so simple, due to how bind_call() handles cast()'s signature

Oh! Yeah, this is because we don't understand overloads, so we treat it as a function defined with an unknown decorator, meaning we currently replace its signature with the "todo signature" (*args: Todo, **kwargs: Todo) -> Todo. So then both positional arguments are bound to the *args parameter.

Short of supporting overloads, which will be a somewhat bigger feature (though we should do it soon), I think the easiest workaround here is to change our definition of the todo signature to (first: Todo = Todo, *args: Todo, **kwargs: Todo) -> Todo. That is, add a first positional argument with default. This signature still accepts any call, but it distinguishes the first parameter such that we can pull out its type separately from call binding.

[carljm]

Should there be a "strict casting" mode

I wouldn't add that now. We can certainly consider it as a feature later, but it's not a "table stakes" feature that we need in order to be usable. It's an opinionated extension of the way cast is specified and works in mypy and pyright.

[KotlinIsland]

basedmypy also reports errors with non-overlapping casts
https://mypy-play.net/?mypy=basedmypy-latest&python=3.12&gist=4642a46d5933b8aa9628918383597bc3