X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fcompiler%2Ftypecheck%2FTcMType.lhs;h=0364a0ed52cc849a5251cb644717b3c0c7e0646e;hb=7ff78c251654969f9973c087bec3ae98d5e5b606;hp=daf2acf3f86c6b416b49f803d517d598e6d16c2d;hpb=9c7f3c9fbe8bd63c8802a87ef8ac0285434dc807;p=ghc-hetmet.git

diff --git a/ghc/compiler/typecheck/TcMType.lhs b/ghc/compiler/typecheck/TcMType.lhs
index daf2acf..0364a0e 100644
--- a/ghc/compiler/typecheck/TcMType.lhs
+++ b/ghc/compiler/typecheck/TcMType.lhs
@@ -50,7 +50,7 @@ import TypeRep		( Type(..), SourceType(..), TyNote(..),	 -- Friend; can see repr
 			) 
 import TcType		( TcType, TcThetaType, TcTauType, TcPredType,
 			  TcTyVarSet, TcKind, TcTyVar, TyVarDetails(..),
-			  tcEqType, tcCmpPred,
+			  tcEqType, tcCmpPred, isClassPred,
 			  tcSplitPhiTy, tcSplitPredTy_maybe, tcSplitAppTy_maybe, 
 			  tcSplitTyConApp_maybe, tcSplitForAllTys,
 			  tcIsTyVarTy, tcSplitSigmaTy, 
@@ -908,7 +908,14 @@ checkAmbiguity forall_tyvars theta tau_tyvars
   where
     complain pred     = addErrTc (ambigErr pred)
     extended_tau_vars = grow theta tau_tyvars
-    is_ambig pred     = any ambig_var (varSetElems (tyVarsOfPred pred))
+
+	-- Only a *class* predicate can give rise to ambiguity
+	-- An *implicit parameter* cannot.  For example:
+	--	foo :: (?x :: [a]) => Int
+	--	foo = length ?x
+	-- is fine.  The call site will suppply a particular 'x'
+    is_ambig pred     = isClassPred  pred &&
+			any ambig_var (varSetElems (tyVarsOfPred pred))
 
     ambig_var ct_var  = (ct_var `elem` forall_tyvars) &&
 		        not (ct_var `elemVarSet` extended_tau_vars)