[GHC] #4807: Data instance for Data.Map is incomplete

[GHC]

#4807: Data instance for Data.Map is incomplete
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    Reporter:  sclv              |       Owner:                             
        Type:  bug               |      Status:  new                        
    Priority:  normal            |   Component:  libraries/base             
     Version:  7.0.1             |    Keywords:                             
    Testcase:                    |   Blockedby:                             
          Os:  Unknown/Multiple  |    Blocking:                             
Architecture:  Unknown/Multiple  |     Failure:  Incorrect result at runtime
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Comment(by dreixel):

 I wouldn't know how to write dataCast1 for Map, but I also don't quite see
 the use case very well ("its certainly easier to use ext1 for partially
 specialized maps than it is to use ext2 and enforce some of the
 specialization by hand"). Can you give me a concrete example?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4807#comment:3>
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