Unpopular opinion: I think it should be "typeclass" and not "type class". The former seems to be way more common in regular usage, despite the fact that the latter is the officially correct spelling (I believe originating with SPJ).
In general, I would use a single word for cases where the thing being described has a more precise (or even different) meaning to what could be inferred from the adjectival word independently modifying the nounal word. "Type[ ]class" is very borderline by this criterion.
