[GHC] #4170: Invalid "duplicate instance" error

[GHC]

#4170: Invalid "duplicate instance" error
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    Reporter:  verdelyi          |       Owner:              
        Type:  bug               |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.3            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
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Comment(by Ashley Yakeley):

 This just bit me too. I need this because I need a way to let the
 programmer tell my stuff about existing instances, since there's currently
 no way to query the "instance environment".

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4170#comment:1>
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