[GHC] #12399: DeriveFunctor fail

[GHC]

#12399: DeriveFunctor fail
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        Reporter:  osa1              |                Owner:
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2404
       Wiki Page:                    |
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Changes (by RyanGlScott):

 * status:  new => patch
 * differential:   => Phab:D2404


Comment:

 Nice catch! I had no idea that `RuntimeRep`-kinded type parameters were
 also represented as actual arguments to a constructor... in any case, your
 intuition that we need to drop the `RuntimeRep` args was spot on.

 (Hopefully, this would also fix derived `Functor` instances for datatypes
 that contain unboxed sums, but I'm not 100% sure on that.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12399#comment:2>
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