Error with ParamSpec when it is in a Union type

Describe the bug

pyright does not process ParamSpec properly when it is in a Union type.

Code or Screenshots

from collections.abc import Callable
from typing import Concatenate, Literal, ParamSpec

P = ParamSpec("P")


def optr(
    num: int,
    fun: Callable[Concatenate[int, P], int] | Literal["plus1"] = "plus1",
    *args: P.args,
    **kwargs: P.kwargs,
) -> int:
    if fun == "plus1":
        return num + 1
    return fun(num, *args, **kwargs)


optr(1)  # pyright gives error: Arguments for ParamSpec "P@optr" are missing (reportCallIssue). mypy is fine with it.

VS Code extension or command-line

Command line, pyright 1.1.407

The issue arose from pandas-dev/pandas-stubs#1526.

I think pyright's behavior here is correct. The value of P goes unsolved in your example.

The correct way to annotate this function is with an overload.

@overload
def optr(num: int, fun: Literal["plus1"] = "plus1") -> int: ...
@overload
def optr[**P](
    num: int,
    fun: Callable[Concatenate[int, P], int],
    *args: P.args,
    **kwargs: P.kwargs,
) -> int: ...
def optr(
    num: int,
    fun: Callable[Concatenate[int, ...], int] | Literal["plus1"] = "plus1",
    *args: Any,
    **kwargs: Any,
) -> int:
    if fun == "plus1":
        return num + 1
    return fun(num, *args, **kwargs)

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Error with `ParamSpec` when it is in a `Union` type #11152

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Error with ParamSpec when it is in a Union type #11152

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Describe the bug

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erictraut commented on Dec 4, 2025

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Error with `ParamSpec` when it is in a `Union` type #11152