-newTyConInstRhs tycon tys =
- let (tvs, ty) = newTyConRhs tycon in substTyWith tvs tys ty
+-- Unwrap one 'layer' of newtype
+-- Use the eta'd version if possible
+newTyConInstRhs tycon tys
+ = ASSERT2( equalLength tvs tys1, ppr tycon $$ ppr tys $$ ppr tvs )
+ mkAppTys (substTyWith tvs tys1 ty) tys2
+ where
+ (tvs, ty) = newTyConEtadRhs tycon
+ (tys1, tys2) = splitAtList tvs tys