X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Ftypecheck%2FTcType.lhs;h=50ac35ab50a97d0a65ac6d795704440ec82d7638;hp=b2da9f04a8eb4ae90f423fa47af80813633466a1;hb=4d8c7c976104d2e39a1183967ec0f254a0fc0a47;hpb=5723262f616ac02ddf637f6ff480a599c737ea0d

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index b2da9f0..50ac35a 100644
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -28,7 +28,7 @@ module TcType (
   MetaDetails(Flexi, Indirect), MetaInfo(..), 
   SkolemInfo(..), pprSkolTvBinding, pprSkolInfo,
   isImmutableTyVar, isSkolemTyVar, isMetaTyVar,  isMetaTyVarTy,
-  isSigTyVar, isExistentialTyVar,  isTyConableTyVar,
+  isSigTyVar, isOverlappableTyVar,  isTyConableTyVar,
   metaTvRef, 
   isFlexi, isIndirect, isUnkSkol, isRuntimeUnkSkol,
 
@@ -614,7 +614,7 @@ isImmutableTyVar tv
   | isTcTyVar tv = isSkolemTyVar tv
   | otherwise    = True
 
-isTyConableTyVar, isSkolemTyVar, isExistentialTyVar, 
+isTyConableTyVar, isSkolemTyVar, isOverlappableTyVar,
   isMetaTyVar :: TcTyVar -> Bool 
 
 isTyConableTyVar tv	
@@ -633,11 +633,14 @@ isSkolemTyVar tv
         FlatSkol {} -> True
  	MetaTv {}   -> False
 
-isExistentialTyVar tv 	-- Existential type variable, bound by a pattern
+-- isOverlappableTyVar has a unique purpose.
+-- See Note [Binding when looking up instances] in InstEnv.
+isOverlappableTyVar tv
   = ASSERT( isTcTyVar tv )
     case tcTyVarDetails tv of
-	SkolemTv (PatSkol {}) -> True
-	_                     -> False
+        SkolemTv (PatSkol {})  -> True
+        SkolemTv (InstSkol {}) -> True
+        _                      -> False
 
 isMetaTyVar tv 
   = ASSERT2( isTcTyVar tv, ppr tv )
@@ -953,6 +956,9 @@ tcInstHeadTyAppAllTyVars :: Type -> Bool
 -- Used in Haskell-98 mode, for the argument types of an instance head
 -- These must be a constructor applied to type variable arguments
 tcInstHeadTyAppAllTyVars ty
+  | Just ty' <- tcView ty       -- Look through synonyms
+  = tcInstHeadTyAppAllTyVars ty'
+  | otherwise
   = case ty of
 	TyConApp _ tys  -> ok tys
 	FunTy arg res   -> ok [arg, res]