[GHC] #5516: Universally quantified GADT context leads to overlapping instance

[GHC]

#5516: Universally quantified GADT context leads to overlapping instance
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    Reporter:  andersk           |        Owner:                           
        Type:  bug               |       Status:  new                      
    Priority:  normal            |    Milestone:                           
   Component:  Compiler          |      Version:  7.3                      
    Keywords:                    |     Testcase:                           
   Blockedby:                    |   Difficulty:                           
          Os:  Unknown/Multiple  |     Blocking:                           
Architecture:  Unknown/Multiple  |      Failure:  GHC rejects valid program
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Comment(by andersk):

 Even if GHC is unable to use the polymorphic evidence, I feel like its
 presence should not cause an error when the code would otherwise have
 worked.  If that’s true, this is a regression from 7.0.  But probably
 nobody writes code like this, so I guess the question is whether there’s
 now a good reason to disallow it.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5516#comment:3>
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