fix of wanted equational class context

Previously failed to account for equational
class context for wanted dictionary contraints, e.g. wanted C a
in

	class a ~ Int => C a
	instance C Int

should give rise to wanted a ~ Int and consequently discharge a ~ Int by
unifying a with Int and then discharge C Int with the instance.

All ancestor equalities are taken into account.

diff --git a/compiler/typecheck/Inst.lhs b/compiler/typecheck/Inst.lhs
index 8d21d1b..e175951 100644 (file)
--- a/compiler/typecheck/Inst.lhs
+++ b/compiler/typecheck/Inst.lhs
@@ -48,7 +48,7 @@ module Inst (
 
        mkWantedCo, mkGivenCo,
        fromWantedCo, fromGivenCo,
-       eitherEqInst, mkEqInst, mkEqInsts,
+       eitherEqInst, mkEqInst, mkEqInsts, mkWantedEqInst,
        finalizeEqInst, writeWantedCoercion,
        eqInstType, updateEqInstCoercion,
        eqInstCoercion,
@@ -1004,6 +1004,12 @@ mkEqInst (EqPred ty1 ty2) co
             }
        where mkName uniq src_span = mkInternalName uniq (mkVarOcc "co") src_span
 
+mkWantedEqInst :: PredType -> TcM Inst
+mkWantedEqInst pred@(EqPred ty1 ty2)
+  = do { cotv <- newMetaTyVar TauTv (mkCoKind ty1 ty2)
+       ; mkEqInst pred (Left cotv)
+       }
+
 -- type inference:
 --     We want to promote the wanted EqInst to a given EqInst
 --     in the signature context.

diff --git a/compiler/typecheck/TcSimplify.lhs b/compiler/typecheck/TcSimplify.lhs
index 007a717..13a85ab 100644 (file)
--- a/compiler/typecheck/TcSimplify.lhs
+++ b/compiler/typecheck/TcSimplify.lhs
@@ -1680,7 +1680,11 @@ reduceContext env wanteds
 
        ; let givens                    = red_givens env
              (given_eqs0,given_dicts0) = partitionGivenEqInsts  givens
-             (wanted_eqs,wanted_dicts) = partitionWantedEqInsts wanteds
+             (wanted_eqs0,wanted_dicts) = partitionWantedEqInsts wanteds
+
+       ; wanted_ancestor_eqs <- (mapM wantedAncestorEqualities wanted_dicts >>= \ls -> return (concat ls))
+       ; traceTc (text "test wanted SCs" <+> ppr wanted_ancestor_eqs)
+       ; let wanted_eqs = wanted_ancestor_eqs ++ wanted_eqs0
 
        ; -- 1. Normalise the *given* *equality* constraints
          (given_eqs,eliminate_skolems) <- normaliseGivens given_eqs0
@@ -2476,6 +2480,31 @@ addSCs is_loop avails dict
     is_given sc_dict = case findAvail avails sc_dict of
                          Just (Given _) -> True        -- Given is cheaper than superclass selection
                          other          -> False       
+
+
+wantedAncestorEqualities :: Inst -> TcM [Inst]
+wantedAncestorEqualities dict 
+  | isClassDict dict
+  = mapM mkWantedEqInst $ filter isEqPred $ bagToList $ wantedAncestorEqualities' (dictPred dict) emptyBag
+  | otherwise
+  = return []
+
+wantedAncestorEqualities' :: PredType -> Bag PredType -> Bag PredType
+wantedAncestorEqualities' pred bag
+  = ASSERT( isClassPred pred )
+    let (clas, tys)             = getClassPredTys pred 
+        (tyvars, sc_theta, _, _) = classBigSig clas
+        sc_theta'               = substTheta (zipTopTvSubst tyvars tys) sc_theta
+        add_sc bag sc_pred
+          | elemBag sc_pred bag  = bag
+         | not (isEqPred sc_pred)
+            && not (isClassPred sc_pred)
+                                = bag
+         | isEqPred sc_pred     = consBag sc_pred bag
+          | otherwise           = let bag' = consBag sc_pred bag
+                                  in wantedAncestorEqualities' sc_pred bag'
+    in foldl add_sc bag sc_theta'
+
 \end{code}
 
 %************************************************************************