1 files changed, 40 insertions, 0 deletions
diff --git a/testsuite/tests/indexed-types/should_fail/T4093b.hs b/testsuite/tests/indexed-types/should_fail/T4093b.hs
new file mode 100644
index 0000000000..2d9878541f
--- /dev/null
+++ b/testsuite/tests/indexed-types/should_fail/T4093b.hs
@@ -0,0 +1,40 @@
+{-# LANGUAGE GADTs, EmptyDataDecls, ScopedTypeVariables, TypeFamilies #-}
+
+module T4093b where
+
+data C
+data O
+
+type family   EitherCO e a b :: *
+type instance EitherCO C a b = a
+type instance EitherCO O a b = b
+
+data MaybeC ex t where
+  JustC    :: t -> MaybeC C t
+  NothingC ::      MaybeC O t
+
+data Block (n :: * -> * -> *) e x
+
+
+blockToNodeList ::
+  forall n e x. (EitherCO e (A C O n) (A O O n) ~ A e O n,
+                 EitherCO x (A C C n) (A C O n) ~ A C x n) =>
+    Block n e x -> A e x n
+
+type A e x n = (MaybeC e (n C O), MaybeC x (n O C))
+blockToNodeList b = foldBlockNodesF (f, l) b z 
+  where
+    z :: EitherCO e (EitherCO e (A C O n) (A O O n)) (EitherCO e (A C O n) (A O O n))
+    z = undefined
+
+    f :: n C O -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C O n) (A O O n)
+    f n _ = (JustC n, NothingC)
+
+    l :: n O C -> EitherCO e (A C O n) (A O O n) -> EitherCO e (A C C n) (A O C n)
+    l _ = undefined
+
+foldBlockNodesF  :: forall n a b c e x .
+                   ( n C O       -> a -> b
+                   , n O C       -> b -> c)
+                 -> (Block n e x -> EitherCO e a b -> EitherCO x c b)
+foldBlockNodesF _ = undefined