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diff --git a/testsuite/tests/indexed-types/should_fail/T2664.hs b/testsuite/tests/indexed-types/should_fail/T2664.hs
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+{-# LANGUAGE EmptyDataDecls, TypeFamilies, TypeOperators, ScopedTypeVariables #-}
+module Overflow where
+import Control.Concurrent
+
+data (:*:) a b
+data (:+:) a b
+
+data family PChan a
+data    instance PChan (a :+: b) = E (IO (PChan a)) (IO (PChan b))
+newtype instance PChan (a :*: b) = O (IO (Either (PChan a) (PChan b)))
+
+type family Dual a
+type instance Dual (a :+: b) = Dual a :*: Dual b
+type instance Dual (a :*: b) = Dual a :+: Dual b
+
+class Connect s where
+    newPChan :: (s ~ Dual c, c ~ Dual s) => IO (PChan s, PChan c)
+
+pchoose :: (t -> a) -> MVar a -> IO (t,b) -> IO b
+pchoose = undefined
+
+instance (Connect a, Connect b) => Connect (a :*: b) where
+    newPChan = do
+        v <- newEmptyMVar
+
+        -- This version is in T2664a
+        -- correct implementation:
+        -- return (O $ takeMVar v, E (pchoose Left v newPChan) (pchoose Right v newPChan))
+       
+        -- type error leads to stack overflow (even without UndecidableInstances!)
+        return (O $ takeMVar v, E (pchoose Right v newPChan) (pchoose Left v newPChan))