Of course it is. "Roast" is most definitely the adjective form of having roasted something, and it is the marketeers (and other illiterates) who have decided "roasted" (as an adjective) sounds more up-market.
I will roast these potatoes (future)
I am roasting these potatoes (present)
I have roasted these potatoes (past transitive)
These potatoes have been roasted (past intransitive)
These are roast potatoes (adjective)
I don't know how far that goes back historically, maybe "roasted" as an adjective was the norm for Sam Pepys or whoever. I'm sure somebody will say...
I really need to gen up on definitions of grammar - what "future perfect" is, for example!