[GHC] #8678: Deriving `Functor` complains about existential type (was: Derivin `Functor` complains about existential type)

[GHC]

#8678: Deriving `Functor` complains about existential type
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        Reporter:  heisenbug         |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.7
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8678#comment:5>
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