To my surprise, GHC does not parse the type `(# | | #)`. I expected this to be the unboxed 3-sum, of kind `TYPE rep1 -> TYPE rep2 -> TYPE rep3 -> TYPE (SumRep [rep1, rep2, rep3])`. I suppose this isn't a bug, per se, as the manual does not suggest GHC would parse this. But it is very odd that there is no way to say this, short of `$( return (UnboxedSumT 3) )` (which does work fine).