Incompatible return type for wrappers around ParamSpec-generics

[BvB93]

Bug Report

When creating a helper function around a ParamSpec-based generic I recently stumbled upon a spurious "Incompatible return value type" error.

Mypy seems to (internally) represent CustomCallable[~P]-parametrized generics as CustomCallable[Callable[P, None]] instead of just CustomCallable[P]. This internal representation can be exposed in certain situations though, resulting in false positives such as in the example below.

To Reproduce

from collections.abc import Callable
from typing import Generic, Any
from typing_extensions import ParamSpec

P = ParamSpec("P")


class CustomCallable(Generic[P]):
    def __init__(self, func: Callable[P, Any]) -> None: ...
    def __call__(self, *args: P.args, **kwargs: P.kwargs) -> None: ...


def func(func: Callable[P, Any]) -> CustomCallable[P]:
    # error: Incompatible return value type (got "CustomCallable[Callable[P, None]]", expected "CustomCallable[P]")
    return CustomCallable(func)

Your Environment

• Mypy version used: mypy 0.931
• Mypy command-line flags: n.a.
• Mypy configuration options from mypy.ini (and other config files): n.a.
• Python version used: 3.10.2
• Operating system and version: Windows 10 (build 19044.1526)

[cdce8p]

/CC: @A5rocks

[A5rocks]

Yeah I wish mypy did not represent P as (...) -> None (I added an alternate representation for concatenate and paramspec literals :-)...

This is definitely a bug that should be solved though. I'll add it to my todo list for after my current paramspec PR is merged :^)

(Unless you want to give this a try: Callable[P, None] should be reducable to P via CallableType.param_spec(), unless I'm misremembering the method.)