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diff --git a/testsuite/tests/gadt/Gadt17_help.hs b/testsuite/tests/gadt/Gadt17_help.hs
new file mode 100644
index 0000000000..30b57133d5
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+++ b/testsuite/tests/gadt/Gadt17_help.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE GADTs #-}
+{-# OPTIONS_GHC -O #-}
+
+module Gadt17_help (
+      TernOp (..), applyTernOp
+    ) where
+
+data TypeWitness a   where
+    TWInt    ::         TypeWitness Int
+    TWBool   ::         TypeWitness Bool
+    TWFloat  ::         TypeWitness Float
+    TWDouble ::         TypeWitness Double
+
+instance (Eq a) => Eq (TypeWitness a) where
+  (==) TWInt     TWInt     = True
+  (==) TWBool    TWBool    = True
+  (==) TWFloat   TWFloat   = True
+  (==) TWDouble  TWDouble  = True
+  (==) _         _         = False
+
+data TernOp a b c d where
+  OpIf       ::                   TypeWitness a                    ->    TernOp Bool   a      a      a
+  OpTernFunc ::  TypeWitness a -> TypeWitness b -> TypeWitness c
+                            -> TypeWitness d -> (a -> b -> c -> d) ->    TernOp a      b      c      d
+
+instance Show (TernOp a b c d) where
+  show (OpIf       {})  = "OpIf"
+  show (OpTernFunc {})  = "OpTernFunc <function>"
+
+
+applyTernOp :: TernOp a b c d -> a -> b -> c -> d
+applyTernOp (OpIf       {})         cond x y = if (cond) then x else y
+applyTernOp (OpTernFunc _ _ _ _ f)  x    y z = f x y z
+