Assert

compiler/types/Type.lhs

@@ -468,7 +468,8 @@ splitFunTys ty = split [] ty ty
 splitFunTysN :: Int -> Type -> ([Type], Type)
 -- ^ Split off exactly the given number argument types, and panics if that is not possible
 splitFunTysN 0 ty = ([], ty)
-splitFunTysN n ty = case splitFunTy ty of { (arg, res) ->
+splitFunTysN n ty = ASSERT2( isFunTy ty, int n <+> ppr ty )
+                    case splitFunTy ty of { (arg, res) ->
 		    case splitFunTysN (n-1) res of { (args, res) ->
 		    (arg:args, res) }}