> In this case, do we need to make this a `TypeError`? Right now you can use `Concatenate` as the return type.

Last I recall, we stopped checking anything in the param args of Callable since 3.10 because it has too many problems with inter-version typing and typing_extensions usage. E.g. in Python 3.8

Callable[typing_extensions.Concatenate[...], ...]

was extremely difficult to implement in typing_extensions since Callable checks were too strict back then (they only allowed list of args). So to avoid such problems in the future, we ceased all param args checking.

For the return arg, I'm not too keen on adding more checks for the same reason of allowing future additions. Also the static type checker should warn the user and mypy correctly does that.