[GHC] #14490: TTG Snags

[GHC]

#14490: TTG Snags
-------------------------------------+-------------------------------------
        Reporter:  alanz             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14482            |  Differential Rev(s):
       Wiki Page:                    |
  ImplementingTreesThatGrow          |
-------------------------------------+-------------------------------------

Comment (by alanz):

 I am not sure if I misunderstand Plan F, but GHC insists on a `Data`
 instance for the `x` in

 {{{#!hs
 newtype GhcExt x = Ext x

 instance (Data x) => Data (GhcExt x) where
   gmapM _f x = return x
   gunfold k z c = case constrIndex c of
                     1 -> k (z Ext)
                     _ -> panic "GhcExt.gunfold"
   dataTypeOf _ = mkNoRepType "HsExtension.GhcExt"
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14490#comment:39>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler