You can write (# Int | Char #), but not (# | #) Int Char. This is annoying since it prevents you from partially applying unboxed sum type constructors, and it also precludes you from doing cool things like reify ''(#||#) (as I woefully noted here).

Luckily, I don't think fixing this would be too hard. The special case of parsing unboxed tuple type constructors as prefix is handled here, so I //think// we'd just need to add a similar case for unboxed sums.