[red-knot] Implement subtyping/assignability between different callable objects

[dhruvmanila]

With #16804 completed, we have the necessary infrastructure to check subtype/assignability relationship between two callable objects. Currently, it only does the check for subtyping/assignability between two GeneralCallableType types, which effectively means typing.Callable annotation or lambdas, as those are the only things that construct the GeneralCallableType. (We do expose knot_extensions.CallableTypeFromFunction from knot_extensions which converts a FunctionType into a GeneralCallableType but that's mainly to help write tests for it.)

This issue is to check other subtype and assignability relationships involving callable objects that still need to be implemented:

• A CallableType is always a subtype of KnownClass::Object (and never the other way around)
• A FunctionType can be a subtype of Callable (but never the other way around)
• Other (non-GeneralCallable) Type::Callable variants, like bound methods, can be subtypes of / assignable to Callables.
• InstanceType (with a __call__ method) can be a subtype of Callable (but never the other way around)

[MatthewMckee4]

A CallableType is always a subtype of KnownClass::Object (and never the other way around)

Can we not generalize this for every type?

[carljm]

Can we not generalize this for every type?

Yes. I actually thought we had already, but maybe it was removed at some point.

[MatthewMckee4]

We have it in is_assignable_to