Fix tcInstHeadTyNotSynonym

It was returning False for type variables amongst other things, so
"instance C a" was telling us to use -XTypeSynonymInstances.

diff --git a/compiler/typecheck/TcType.lhs b/compiler/typecheck/TcType.lhs
index 7595a75..eaf2faa 100644 (file)
--- a/compiler/typecheck/TcType.lhs
+++ b/compiler/typecheck/TcType.lhs
@@ -797,10 +797,9 @@ tcInstHeadTyNotSynonym :: Type -> Bool
 -- are transparent, so we need a special function here
 tcInstHeadTyNotSynonym ty
   = case ty of
-       NoteTy _ ty     -> tcInstHeadTyNotSynonym ty
-       TyConApp tc tys -> not (isSynTyCon tc)
-       FunTy arg res   -> True
-       other           -> False
+        NoteTy _ ty     -> tcInstHeadTyNotSynonym ty
+        TyConApp tc tys -> not (isSynTyCon tc)
+        _ -> True
 
 tcInstHeadTyAppAllTyVars :: Type -> Bool
 -- Used in Haskell-98 mode, for the argument types of an instance head