The inverse of #48 is expanding into ellipsis.

I think this should use the same syntax as #48

args <- list(sep = "-", collapse = "; ")
paste("Hello", c("World", "Friends"), ..args)

I don't think there are situations where it's ambiguous. ..rest for collecting into arguments will only happen when destructuring or in a function signature, while ..rest for passing arguments only happens in calls. This might introduce some confusion in situations where the ..rest argument in a function signature is re-used as an argument to default value for another parameter:

f <- function(a, b = sum(..rest), ..rest)) {
}

In this case, the ..rest at the end of the function call collects args, while the ..rest in names(..rest) passes those arguments to sum.