diff --git a/ghc/compiler/hsSyn/HsDecls.lhs b/ghc/compiler/hsSyn/HsDecls.lhs
index 1c3c8d41fe8bff55766aad113d9293a03c793378..4e6c2b422000d1947dbbde93e32f044bb173ac7d 100644
--- a/ghc/compiler/hsSyn/HsDecls.lhs
+++ b/ghc/compiler/hsSyn/HsDecls.lhs
@@ -185,6 +185,9 @@ instance Ord name => Eq (TyClDecl name pat) where
 	  eqListBy (eq_cls_sig env) sigs1 sigs2
        )
 
+  (==) _ _ = False	-- default case
+
+
 eq_hsFD env (ns1,ms1) (ns2,ms2)
   = eqListBy (eq_hsVar env) ns1 ns2 && eqListBy (eq_hsVar env) ms1 ms2