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diff --git a/testsuite/tests/arrows/gadt/T17423.hs b/testsuite/tests/arrows/gadt/T17423.hs
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+{-# LANGUAGE Arrows, GADTs #-}
+
+module Main where
+
+import Control.Arrow
+import Control.Category
+import Prelude hiding (id, (.))
+
+data DecoType a where
+   -- | Icons and colours for @False@ and @True@ respectively.
+   DecoBool :: Maybe (String, String) -> Maybe (Int, Int) -> DecoType Bool
+   -- | Icons and colours for ranges within type @a@.
+   DecoRange :: String -> DecoType a
+
+-- Sub-dialog for designing decorated booleans.
+decoBoolDialog :: Gadget (DecoType Bool) (DecoType Bool)
+decoBoolDialog =
+      -- arr (\(DecoBool i c) -> (i, c)) >>> (icons *** colours) >>> arr (uncurry DecoBool)
+      proc (DecoBool i c) -> do   -- Compiler panic in GHC 8.6.5.
+         i1 <- id -< i
+         c1 <- id -< c
+         returnA -< DecoBool i1 c1
+
+
+
+data Gadget b c = Pure (b -> c)
+
+instance Category Gadget where
+   id = Pure id
+   Pure g1 . Pure g2 = Pure $ g1 . g2
+
+instance Arrow Gadget where
+   arr = Pure
+   first (Pure f) = Pure $ \(b, b1) -> (f b, b1)
+
+
+main = putStrLn "Hello world."