[GHC] #3927: Incomplete/overlapped pattern warnings + GADTs = inadequate

[GHC]

#3927: Incomplete/overlapped pattern warnings + GADTs = inadequate
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    Reporter:  simonpj           |        Owner:  simonpj     
        Type:  bug               |       Status:  new         
    Priority:  high              |    Milestone:  7.2.1       
   Component:  Compiler          |      Version:  6.12.1      
    Keywords:                    |     Testcase:              
   Blockedby:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Blocking:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
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Comment(by sjoerd_visscher):

 Here's another case in code from Edward Kmett: [https://github.com/ekmett
 /semigroupoid-extras/blob/master/Data/Semigroupoid/Coproduct.hs#L17]

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3927#comment:18>
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