s3_dispatch error when using namespace scoping

[TylerGrantSmith]

sloop::s3_dispatch(vctrs::vec_arith("+", 2, 3))
#> Error: Mapped vectors must have consistent lengths:
#> * `.x` has length 3
#> * `.y` has length 2

Not sure if it is better to handle this case specially or to modify the check

 if (!is_call(call)) {
        stop("`call` must be a function call", call. = FALSE)
    }