Is literal ellipsis really valid as ParamSpec last argument?

Question

Quote from Python docs for Concatenate :

The last parameter to Concatenate must be a ParamSpec or ellipsis (...).

I know what ParamSpec is, but the ellipsis here drives me mad. It is not accepted by mypy :

from typing import Callable, ParamSpec, Concatenate, TypeVar, Generic

_P = ParamSpec('_P')
_T = TypeVar('_T')


class Test(Generic[_P, _T]):
    fn: Callable[Concatenate[_P, ...], _T]

E: Unexpected "..." [misc]
E: The last parameter to Concatenate needs to be a ParamSpec  [valid-type]

and is not explained anywhere in docs. PEP612 doesn't mention it. Is it just a mistake, appeared as a result of mixing Callable and Concatenate together?

This issue is somewhat related and shows syntax with ellipsis literal in Concatenate :

The specification should be extended to allow either Concatenate[int, str, ...] , or [int, str, ...] , or some other syntax.

But this clearly targets "future syntax".

Note: I'm aware of meaning of ellipsis as Callable argument, this question is specifically about Concatenate .

Answer 1

According to PEP-612's grammar , the ellipsis is not permitted in the Concatenate expression:

We now augment that with two new options: a parameter specification variable ( Callable[P, int] ) or a concatenation on a parameter specification variable ( Callable[Concatenate[int, P], int] ).

callable ::= Callable "[" parameters_expression, type_expression "]"

parameters_expression ::=
  | "..."
  | "[" [ type_expression ("," type_expression)* ] "]"
  | parameter_specification_variable
  | concatenate "["
                   type_expression ("," type_expression)* ","
                   parameter_specification_variable
                "]"

where parameter_specification_variable is a typing.ParamSpec variable, declared in the manner as defined above, and concatenate is typing.Concatenate .

However, the support for ellipsis as the last argument for Concatenate was introduced in April 2022 as part of Python 3.11.

No type checker seems to handle this new case though.