So I have something that looks similar to http://hackage.haskell.org/trac/ghc/ticket/5252, namely, given these two modules: % cat Midi.hs {-# OPTIONS_GHC -funbox-strict-fields #-} module Midi ( WriteMessage(..) , WriteDevice -- TODO due ghc bug: http://hackage.haskell.org/trac/ghc/ticket/5252 -- , WriteDevice(WriteDevice) ) where import qualified Data.ByteString as ByteString data WriteMessage = WriteMessage !WriteDevice newtype WriteDevice = WriteDevice ByteString.ByteString % cat CoreMidi.hs module CoreMidi where import qualified Midi write_message :: Midi.WriteMessage -> IO Bool write_message (Midi.WriteMessage _) = return True % ghc -c Midi.hs % ghc -c CoreMidi.hs ghc: panic! (the 'impossible' happened) (GHC version 7.6.1 for x86_64-apple-darwin): reboxProduct: not a product main:Midi.WriteDevice{tc r2M} Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug Oddly, if I put {-# UNPACK #-} on the strict WriteDevice and remove -funbox-strict-fields, I don't get a crash anymore. Also, it has to be ByteString inside, I guess it has to do with the optimization ByteString applies. Shall I append to the old ticket, or create a new one?