Ticket #3954 (closed bug: duplicate)
Contexts on empty data decls falsely rejected
HList uses EmptyDataDecls? and contains
data HNat x => Label x ns desc
which fails with
Data/HList/Label3.hs:37:0: No context is allowed on a GADT-style data declaration (You can put a context on each contructor, though.)
, regressing from 6.10.
My crystal ball tells me someone has been simplifying the implementation of EmptyDataDecls? to be GADTs without where, and missed the corner case.