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.

[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.

[KotlinIsland]

I'd also like to see an implementation in at least one type checker

I have wip support in PyCharm, but I could also add it to basedpyright

it was very straightforward to implement

[davidhalter]

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.

Oh right, sorry I didn't see them, because I thought they would be in a different file. I'm also very much +1 on putting those tests into a different file (for example generics_paramspec_variance.py).