Fix a bug in MatchCon, and clarify what dataConInstOrigArgTys does

[ghc-hetmet.git] / compiler / typecheck / TcDeriv.lhs

diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index b973ec4..e26c97d 100644 (file)
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -479,7 +479,8 @@ mk_data_eqn loc orig tvs cls tycon tc_args rep_tc rep_tc_args
        ; let ordinary_constraints
                = [ mkClassPred cls [arg_ty] 
                  | data_con <- tyConDataCons rep_tc,
-                   arg_ty   <- dataConInstOrigArgTys data_con rep_tc_args,
+                   arg_ty   <- ASSERT( isVanillaDataCon data_con )
+                               dataConInstOrigArgTys data_con rep_tc_args,
                    not (isUnLiftedType arg_ty) ] -- No constraints for unlifted types?
 
              tiresome_subst = zipTopTvSubst (tyConTyVars rep_tc) rep_tc_args