`mypy` doesn't understand that `*args: P.args` implies that `args` is a `tuple`

[randolf-scholz]

from typing import Callable

def as_tuple[*Ts](*args: *Ts) -> tuple[*Ts]: return args
def tuple_identity[*Ts](t: tuple[*Ts]) -> tuple[*Ts]: return t
def tuple_identity2[T: tuple](t: T) -> T: return t


def test_paramspec[**P](
    dummy: Callable[P, None],  # ensure P is bound
    /,
    *args: P.args,
    **kwargs: P.kwargs,
) -> None:
    reveal_type(args)                   # N: "P.args`-1" 
    reveal_type( (*args,) )             # N: "builtins.tuple[P.args`-1, ...]"
    reveal_type( tuple(args) )          # N: "builtins.tuple[builtins.object, ...]"
    reveal_type(as_tuple(*args))        # N: "builtins.tuple[P.args`-1, ...]"
    reveal_type(tuple_identity(args))   # N: "builtins.tuple[Never, ...]"
                                        # E: [arg-type]
    reveal_type(tuple_identity2(args))  # N: "P.args`-1" ✅

    if isinstance(args, tuple):
        pass
    else:
        reveal_type(args)  # false negative [warn-unreachable]

https://mypy-play.net/?mypy=latest&python=3.12&flags=warn-unreachable&gist=3eabd8f4ee599e272934c94080f02bd8

Ideally, all the reveal_types should show the same result (or raise errors if considered misuse of ParamSpec¹), and the else-branch should trigger an unreachable warning.

Footnotes

1. For instance, pyright says as_tuple(*args) is illegal. Code sample in pyright playground ↩

[sterliakov]

We shouldn't replicate pyright behavior here, it is not a type error but a weirdly phrased "not supported" message (discussion). That usage is perfectly fine and (IMO) should produce a tuple[object, ...].