parameter specification variance

[KotlinIsland]

discussion: https://discuss.python.org/t/parameter-specifications-should-have-variance/106452

[JelleZijlstra]

I like adding this if we can get it specified nicely, but this PR is not ready; the proposed test is incorrect.

Also, https://typing.python.org/en/latest/spec/generics.html#paramspec-variables still says variance on ParamSpec is unsupported; this should be updated.

I'd also like to see an implementation in at least one type checker, even if only as a draft PR, so we can be confident this is something that can be feasibly implemented.

[JelleZijlstra]

This should presumably be using CovariantParamSpecOld, not ContravariantParamSpecOld.

[JelleZijlstra]

This class should not actually be covariant. Callable's argument is a contravariant position, so the variance flips.

You can see that in current type checkers with a simpler example that just uses a TypeVar:

from typing import Callable

class X[T]:
    def meth(self) -> Callable[[T], None]:
        raise NotImplementedError

def take_xint(x: X[int]): ...
def take_xobj(x: X[object]): ...

def f(xobj: X[object], xint: X[int]):
    take_xint(xobj)  # ok
    take_xobj(xint)  # error

X here is contravariant.

A correct covariant class would look like this:

from typing import Callable

class UseCallback[T]:
    def meth(self, callback: Callable[[T], None]):
        raise NotImplementedError

def take_xint(x: UseCallback[int]): ...
def take_xobj(x: UseCallback[object]): ...

def f(xobj: UseCallback[object], xint: UseCallback[int]):
    take_xint(xobj)  # error
    take_xobj(xint)  # ok

[davidhalter]

I personally would also like to see tests added for param spec variance inference, since that would probably need to be supported as well.

[JelleZijlstra]

The test cases do have variance inference, though as I noted some of the cases are wrong. But it would probably be useful to have a few more cases, and I'd recommend putting the tests for paramspec variance in their own file so we can track type checker support more precisely.