X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2FbasicTypes%2FDataCon.lhs;h=458bfd3f8193ac6eca8402344223311ed3f52898;hb=cbebca1c9164a5e5ae9b117d0dcf5ad217defc6d;hp=fae899d4d1d29705be954df58c49cccfd80880d8;hpb=fdf8656855d26105ff36bdd24d41827b05037b91;p=ghc-hetmet.git

diff --git a/compiler/basicTypes/DataCon.lhs b/compiler/basicTypes/DataCon.lhs
index fae899d..458bfd3 100644
--- a/compiler/basicTypes/DataCon.lhs
+++ b/compiler/basicTypes/DataCon.lhs
@@ -300,7 +300,7 @@ data DataCon
 		-- the context constrains only universally quantified variables
 		--	MkT :: forall a b. (a ~ b, Ord b) => a -> T a b
 	dcOtherTheta :: ThetaType,  -- The other constraints in the data con's type
-		                    -- *other than* those in the dcEqSpec
+		                    -- other than those in the dcEqSpec
 
 	dcStupidTheta :: ThetaType,	-- The context of the data type declaration 
 					--	data Eq a => T a = ...
@@ -835,16 +835,17 @@ dataConCannotMatch :: [Type] -> DataCon -> Bool
 -- Returns True iff the data con *definitely cannot* match a 
 --		    scrutinee of type (T tys)
 --		    where T is the type constructor for the data con
---
+-- NB: look at *all* equality constraints, not only those
+--     in dataConEqSpec; see Trac #5168
 dataConCannotMatch tys con
-  | null eq_spec      = False	-- Common
+  | null theta        = False	-- Common
   | all isTyVarTy tys = False	-- Also common
   | otherwise
-  = typesCantMatch (map (substTyVar subst . fst) eq_spec)
-	           (map snd eq_spec)
+  = typesCantMatch [(Type.substTy subst ty1, Type.substTy subst ty2)
+                   | EqPred ty1 ty2 <- theta ]
   where
     dc_tvs  = dataConUnivTyVars con
-    eq_spec = dataConEqSpec con
+    theta   = dataConTheta con
     subst   = zipTopTvSubst dc_tvs tys
 \end{code}