[GHC] #8913: either bug or confusing error message mixing PolyKinds and TypeFamilies

[GHC]

#8913: either bug or confusing error message mixing PolyKinds and TypeFamilies
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        Reporter:  ghorn             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler (Type    |          Version:  7.8.1-rc2
  checker)                           |         Keywords:  PolyKinds,
      Resolution:                    |  TypeFamilies
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 ghorn):

 A workaround for this is to put the "GCat" class in its own module
 (without PolyKinds), and make orphan instances of it where needed (with
 PolyKinds).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8913#comment:3>
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