[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:  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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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"cdac487bcd9928d77738f6e79ead7b9bb4bc00fd/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="cdac487bcd9928d77738f6e79ead7b9bb4bc00fd"
 Make -XDeriveFunctor more generous about non-last arguments (Trac #8678)

 When deriving Functor, Foldable, Traversable, we need only look at the
 way that the last type argument is treated.  It's fine for there to
 be existentials etc, provided they don't affect the last type argument.

 See Note [Check that the type variable is truly universal] in TcDeriv.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8678#comment:6>
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