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diff --git a/testsuite/tests/indexed-types/should_fail/T15870.hs b/testsuite/tests/indexed-types/should_fail/T15870.hs
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+{-# LANGUAGE DataKinds              #-}
+{-# LANGUAGE FlexibleInstances      #-}
+{-# LANGUAGE GADTs                  #-}
+{-# LANGUAGE MultiParamTypeClasses  #-}
+{-# LANGUAGE PolyKinds              #-}
+{-# LANGUAGE ScopedTypeVariables    #-}
+{-# LANGUAGE TypeFamilies           #-}
+{-# LANGUAGE TypeOperators          #-}
+
+module T15870 where
+
+data Optic a where
+  --Index :: Nat    -> Optic a
+  --Name  :: Symbol -> Optic a
+  (:.:) :: Optic a -> Optic b -> Optic a -- composition
+
+class Gettable a (optic :: Optic a) where
+  type Get a (optic :: Optic a)
+
+{-
+some basic instances, e.g.
+instance Gettable (a,b) (Index 0) where
+  type Get (a,b) (Index 0) = a
+...
+-}
+
+instance forall a b (g1 :: Optic a) (g2 :: Optic b).
+        ( Gettable a g1
+        , b ~ Get a g1
+        , Gettable b g2
+        ) => Gettable a (g1 :.: g2) where
+  type Get a (g1 :.: g2) = Get a g2