[GHC] #11728: Core lint errors

[GHC]

#11728: Core lint errors
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        Reporter:  Iceland_jack      |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 I think `Error.hs` has the same problem, uses `UndecidableSuperclasses`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11728#comment:3>
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