`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, ...].