ghc-dup: 8fc11a3b0d6afa9323b94fff5fab7a6b1c52f1ae

     1: import GHC.Dup
     2: import Control.Concurrent.MVar
     3: import Control.Concurrent
     4: 
     5: main = do
     6:     v <- newEmptyMVar :: IO (MVar Int)
     7:     case deepDup (putMVar v 0) of Box a -> a
     8:     readMVar v >>= print