Why does Concatenate only work one way?

[decorator-factory]

What's the rationale behind Concatenate (from PEP 612) only allowing a ParamSpec variable to appear on the right?
i.e. Concatenate[int, P] is legal, but Concatenate[P, int] is not.

I wanted to make a decorator that added authentication and authorization handling on HTTP controllers:

@requires("admin")
async def delete_everything(req: Request, user: AuthenticatedUser) -> Response:
    ... 
# delete_everything becomes:
# async (req: Request) -> Response
# which can be then used as a standard handler in framework, in my case Starlette

Because I wanted it to work on methods as well as with other similar decorators:

@requires("admin")
@parse_body_as(UserMention)
def ban_user(self, mention: UserMention, user: AuthenticatedUser) -> Response:
    ...
# ban_user becomes:
# async (self, req: Request) -> Response
# and then I can use the bound method as a handler

Traditionally things like this have been done without adding arguments, but with adding extra attributes to the Request objects, but that's not possible to express in type hints as far as I understand.

I am aware that this is just how the grammar is defined in the PEP, but maybe there's some deeper reason why what I wanted isn't possible?

This is possible in TypeScript, so I was surprised it doesn't work with PEP 612.

[layday]

It's because Callable only supports expressing positional arguments. If you have Callable[Concatenate[P, int], None], then what should P capture? It can't capture kwargs so that's a limitation that'd have to be imposed on ParamSpec in this context. But also, if you imagine this as def foo(*args, bar: int), what that means is the int would have to be a keyword-only argument, and keyword-only arguments are not supported by Callable.