issue with ParamSpec and Callable of Callables

[graingert]

Bug Report
When using ParamSpec and passing a Callable that takes a Callable, mypy 1.6.0, 1.6.1 and master rejects the input, this is a regression from 1.5.1

To Reproduce

from typing import *

T_Retval = TypeVar("T_Retval")
P = ParamSpec("P")


def run(
    func: Callable[[], T_Retval],
    *args: object,
    backend: str = "asyncio",
) -> T_Retval:
    return func()


def run_portal():
    pass


def submit(
    func: Callable[P, T_Retval],
    /,
    *args: P.args,
    **kwargs: P.kwargs,
) -> T_Retval:
    return func(*args, **kwargs)


submit(
    run,
    run_portal,
    backend="asyncio",
)

https://mypy-play.net/?mypy=1.6.1&python=3.11&gist=e282d121f479fcb77ec89daeb2a07b60

Expected Behavior

Should pass

Actual Behavior

main.py:29: error: Argument 1 to "submit" has incompatible type "Callable[[Callable[[], T_Retval], VarArg(object), DefaultNamedArg(str, 'backend')], T_Retval]"; expected "Callable[[Callable[[], Any], str], T_Retval]"  [arg-type]

Your Environment

• Mypy version used: 1.5.1 (passes) 1.6.1, 1.6.0, master (fails)
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.11

[ilevkivskyi]

This may be caused by #15896, IIRC. It was kind of an intentional trade-of. We added support for some more common cases (e.g. generic apply with positional arguments) knowing this may cause some regressions for more tricky cases (e.g. those involving keyword-only arguments). The decision was to see what use cases people actually need, and add some special-casing accordingly.

Since this is quite a tricky thing, the fix will not be available in 1.7, even though this is technically a regression (unless I will find a low-risk solution), fix will probably be available in 1.8.