also swap for variables in completion algorithm

diff --git a/compiler/typecheck/TcTyFuns.lhs b/compiler/typecheck/TcTyFuns.lhs
index 0130aa6..c91ac63 100644 (file)
--- a/compiler/typecheck/TcTyFuns.lhs
+++ b/compiler/typecheck/TcTyFuns.lhs
@@ -526,7 +526,12 @@ swapInst i@(EqInst {})
                -- we should swap!
              go ty1 ty2@(TyConApp tyCon _) 
                                        | isOpenSynTyCon tyCon
-                                       = do { wg_co <- eitherEqInst i
+                                       = actual_swap ty1 ty2
+             go ty1@(TyConApp _ _) ty2@(TyVarTy _)
+                                       = actual_swap ty1 ty2
+             go _ _                    = return (i,False)
+
+             actual_swap ty1 ty2 = do { wg_co <- eitherEqInst i
                                                          -- old_co := sym new_co
                                                          (\old_covar ->
                                                           do { new_cotv <- newMetaTyVar TauTv (mkCoKind ty2 ty1)
@@ -539,7 +544,6 @@ swapInst i@(EqInst {})
                                             ; new_inst <- mkEqInst (EqPred ty2 ty1) wg_co
                                             ; return (new_inst,True)
                                             }
-             go _ _                    = return (i,False)
 
 -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 decompInsts :: [Inst] -> TcM ([Inst],Bool)