[GHC] #12882: Unexpected constraint when using ExistentialQuantification

[GHC]

#12882: Unexpected constraint when using ExistentialQuantification
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        Reporter:  danilo2           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  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):
       Wiki Page:                    |
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Comment (by mpickering):

 Can you paste your whole file please? It isn't clear which extensions you
 have turned on to get the first example to compile.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12882#comment:1>
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