[GHC] #8740: Deriving instance conditionally compiles

[GHC]

#8740: Deriving instance conditionally compiles
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        Reporter:  thomaseding       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.6.3
  checker)                           |             Keywords:  GADTs,
      Resolution:                    |  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #8128             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by bgamari):

 Indeed I was bitten by this just now. This is quite an unfortunate
 behavior.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8740#comment:5>
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