hs-boot/hsig ambiguity empty data declaration or type with no constructors
When I write in a boot or signature file:
it is ambiguous whether or not this is a fully-specified empty data declaration, or a data type which has its constructors unspecified. This doesn't matter too much for hs-boot (since you can't use the data constructors in any case) but for signatures we might want to force a data type to be empty. Though I think this case should be rare.