*args positional parameter dropped out and became a phantom when using Concatenate to shorten parameters

[NCBM]

Describe the bug

When using typing.Concatenate to reduce parameters (just like a part of partial function), I found the positional parameter *args (or any other name) dropped out of the parameter list after a Concatenate operation and become a phantom in pyright, which is not printed in parameter type while type checking of the *args still works.

I also tested the code in pylance but the error is even not reported.

Code or Screenshots

from typing import Callable, Concatenate, reveal_type


def _onepartial[T, **P, R](fn: Callable[Concatenate[T, P], R], val: T) -> Callable[P, R]:
    ...


def test2(p1: int, *ps: str) -> None:
    ...


reveal_type(_onepartial(test2, 3))
reveal_type(_onepartial(_onepartial(test2, 3), "foo"))  # *ps dropped out
reveal_type(
    _onepartial(
        _onepartial(
            _onepartial(test2, 3),
            "foo"
        ),
        "bar"  # no error since *ps type match, although which becomes a phantom
    )
)
reveal_type(
    _onepartial(
        _onepartial(
            _onepartial(test2, 3),
            "foo"
        ),
        3  # error since *ps type mismatch, although which becomes a phantom
    )
)

View in pyright-play.net

VS Code extension or command-line

Tested in:

• pyright-play.net: pyright 1.1.391, python 3.13;
• vscode: pylance 2024.12.1, python 3.13, however not reporting the error as commented.

[erictraut]

Thanks for the issue. The typing spec isn't entirely clear how a Concatenate should be used in conjunction with a *args parameter, but I agree that your interpretation (which is consistent with mypy's interpretation) makes sense. I'll change pyright's behavior to match. This will be included in the next release.

Pylance doesn't show this error because it defaults typeCheckingMode to "off". If you set this to "standard", you'll see the same errors reported by pyright.

[NCBM]

thanks. maybe I forgot to change the typeCheckingMode.