[GHC] #12801: Misleading error message when deriving Functor

[GHC]

#12801: Misleading error message when deriving Functor
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        Reporter:  erikd             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I think this is a duplicate of #10684, as the failure to derive `Functor`
 for `Wibble` prevents `Eq` and `Show` from being derived for `Wibble`,
 causing a cascade of error messages for `Container`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12801#comment:1>
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