[GHC] #8678: Deriving `Functor` complains about existential type

[GHC]

#8678: Deriving `Functor` complains about existential type
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        Reporter:  heisenbug                     |            Owner:
            Type:  bug                           |           Status:
        Priority:  normal                        |  closed
       Component:  Compiler                      |        Milestone:
      Resolution:  fixed                         |          Version:  7.7
Operating System:  Unknown/Multiple              |         Keywords:
 Type of failure:  None/Unknown                  |     Architecture:
       Test Case:                                |  Unknown/Multiple
  deriving/should_compile/T8678                  |       Difficulty:
        Blocking:                                |  Unknown
                                                 |       Blocked By:
                                                 |  Related Tickets:
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Changes (by simonpj):

 * status:  new => closed
 * testcase:   => deriving/should_compile/T8678
 * resolution:   => fixed


Comment:

 Good idea.  I have liberalise what `deriving Functor` will do, so that it
 looks only at the way in which the last type parameter is used.  This
 makes perfect sense.

 Let's not merge this; it's a new feature.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8678#comment:7>
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