[GHC] #10778: GHC doesn't infer all constrains

[GHC]

#10778: GHC doesn't infer all constrains
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        Reporter:  danilo2           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Simon, I see pretty much the same error unless I enable
 `UndecidableInstances`. Should I enable that for the test, or do we still
 have a problem?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10778#comment:11>
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