Fix #584

[paf31]

@garyb Mind reviewing this one?

[coveralls]

Coverage remained the same when pulling 948b07d on 584 into 94e0c34 on master.

[garyb]

I think I understand why that works (and if so, it looks good to me and makes sense), but mind explaining a bit so I'm sure?

[paf31]

Sure. The $? operator applies the current substitution, and starIfUnknown is something we do with kinds to default them to * if there are any unknowns left, since we don't support polymorphic kinds. If starIfUnknown is used before $? then there could be unknowns left which are only unknown because the substitution hasn't been applied yet, which means they aren't really *.

[garyb]

Great, that's what I thought was happening. I'm just surprised we've not had more problems with kinds already then, I guess * being by far the most common kind means starIfUnknown is usually right though, hiding the problem.