- -- (3) construct representation tycon
- ; rep_tc_name <- newFamInstTyConName tc_name (srcSpanStart loc)
+ ; -- (3) check that the right-hand side is a tau type
+ ; unless (isTauTy t_rhs) $
+ addErr (polyTyErr t_rhs)
+
+ -- (4) construct representation tycon
+ ; rep_tc_name <- newFamInstTyConName tc_name loc