X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypes%2FInstEnv.lhs;h=07f68f7b91ce91c6c271b201f7fdce8ddcb20027;hp=dfbe0a063aa00bc82f1810ce2dff30f86ddb628f;hb=16b9e80dc14db24509f051f294b5b51943285090;hpb=ca9e79e1c70a26b12ea4b63f3a1c5a804462c1a5

diff --git a/compiler/types/InstEnv.lhs b/compiler/types/InstEnv.lhs
index dfbe0a0..07f68f7 100644
--- a/compiler/types/InstEnv.lhs
+++ b/compiler/types/InstEnv.lhs
@@ -128,7 +128,7 @@ setInstanceDFunId ispec dfun
 	-- are ok; hence the assert
      ispec { is_dfun = dfun, is_tvs = mkVarSet tvs, is_tys = tys }
    where 
-     (tvs, _, tys) = tcSplitDFunTy (idType dfun)
+     (tvs, _, _, tys) = tcSplitDFunTy (idType dfun)
 
 instanceRoughTcs :: Instance -> [Maybe Name]
 instanceRoughTcs = is_tcs
@@ -184,7 +184,7 @@ mkLocalInstance dfun oflag
 		is_tvs = mkVarSet tvs, is_tys = tys,
                 is_cls = className cls, is_tcs = roughMatchTcs tys }
   where
-    (tvs, cls, tys) = tcSplitDFunTy (idType dfun)
+    (tvs, _, cls, tys) = tcSplitDFunTy (idType dfun)
 
 mkImportedInstance :: Name -> [Maybe Name]
 		   -> DFunId -> OverlapFlag -> Instance
@@ -195,7 +195,7 @@ mkImportedInstance cls mb_tcs dfun oflag
 		is_tvs = mkVarSet tvs, is_tys = tys,
 		is_cls = cls, is_tcs = mb_tcs }
   where
-    (tvs, _, tys) = tcSplitDFunTy (idType dfun)
+    (tvs, _, _, tys) = tcSplitDFunTy (idType dfun)
 
 roughMatchTcs :: [Type] -> [Maybe Name]
 roughMatchTcs tys = map rough tys
@@ -499,7 +499,7 @@ lookupInstEnv (pkg_ie, home_ie) cls tys
  	find ((item, map (lookup_tv subst) dfun_tvs) : ms) us rest
 
 	-- Does not match, so next check whether the things unify
-	-- See Note [overlapping instances] above
+	-- See Note [Overlapping instances] above
       | Incoherent <- oflag
       = find ms us rest
 
@@ -533,14 +533,18 @@ insert_overlapping new_item (item:items)
     old_beats_new = item `beats` new_item
 
     (instA, _) `beats` (instB, _)
-	= overlap_ok && 
-	  isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
-		-- A beats B if A is more specific than B, and B admits overlap
-		-- I.e. if B can be instantiated to match A
-	where
-	  overlap_ok = case is_flag instB of
-			NoOverlap -> False
-			_         -> True
+          = overlap_ok && 
+            isJust (tcMatchTys (is_tvs instB) (is_tys instB) (is_tys instA))
+                    -- A beats B if A is more specific than B,
+                    -- (ie. if B can be instantiated to match A)
+                    -- and overlap is permitted
+          where
+            -- Overlap permitted if *either* instance permits overlap
+            -- This is a change (Trac #3877, Dec 10). It used to
+            -- require that instB (the less specific one) permitted overlap.
+            overlap_ok = case (is_flag instA, is_flag instB) of
+                              (NoOverlap, NoOverlap) -> False
+                              _                      -> True
 \end{code}