[Regression] Inconsistent error with ParamSpec

[cdce8p]

Bug Report
Notice this while testing the master branch. I bisected the regression to

• Polymorphic inference: support for parameter specifications and lambdas #15837

/CC @ilevkivskyi

To Reproduce

https://mypy-play.net/?mypy=master&python=3.11&gist=b8554f05b565ff448d1a291b3dbc38c3

from collections.abc import Callable
from typing import ParamSpec, Concatenate, TypeVar, Any

_P = ParamSpec("_P")
_R = TypeVar("_R")

class A: ...
a = A()

def a_func(
    func: Callable[Concatenate[A, _P], _R],
) -> Callable[Concatenate[Any, _P], _R]:

    def wrapper(_: Any, /, *args: _P.args, **kwargs: _P.kwargs) -> _R:
        return func(a, *args, **kwargs)

    return wrapper


def filter(a, *args):
    ...


x: Any
y: object

a_func(filter)
x = a_func(filter)  # error
y = a_func(filter)

Expected Behavior
no error

Actual Behavior

main.py:28: error: Argument 1 to "a_func" has incompatible type "Callable[[Any, VarArg(Any)], Any]"; expected "Callable[[A, VarArg(Any), KwArg(Any)], Any]"  [arg-type]
main.py:28: note: This is likely because "filter" has named arguments: "a". Consider marking them positional-only
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.7.0+dev.c0906408c10d24d748711fa24be5befb2c794d4c (compiled: no)
• Python version used: 3.11.5

[ilevkivskyi]

Yeah, it is really bad when switching some precise type to Any results in an error, but here it is kind of expected. There is this thing called infer_against_any (or similar), and its idea is that when we have Any actual, we should infer some very forgiving type. The problem is that inferring Any for _P creates a type that is not forgiving at all (if there is a prefix, because plain *args: Any, **kwargs: Any are special-cased). I guess this issue will be fixed by #15913 in its original form (btw could you please check if you will have time).

I am still hesitating about what to do about that PR, and @JukkaL decided to ignore me there, so I guess I will need to make the hard choice myself.

[cdce8p]

Thanks for the reply! I did check #15913 with Home Assistant (where I also noticed this error). #15913 on it's own not fix it. However, I'm also not seeing any new regressions so should be fine from that POV.

BTW feel free to ping me if I should check a PR against the whole Home Assistant test suite. I've a branch setup to test it quite easily with all it's 1300+ requirements. Might sometimes be a good addition to the regular mypy primer result.