[GHC] #7123: Dictionary method should be applicable even in existential context

[GHC]

#7123: Dictionary method should be applicable even in existential context
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 Reporter:  heisenbug         |          Owner:                  
     Type:  bug               |         Status:  new             
 Priority:  normal            |      Component:  Compiler        
  Version:  7.4.1             |       Keywords:                  
       Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
  Failure:  None/Unknown      |       Testcase:                  
Blockedby:                    |       Blocking:                  
  Related:                    |  
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Comment(by heisenbug):

 I lost my nicely formatted bug report, but I'll try to explain the gist
 here:

   Since the `Quux` has been constructed with the matching dictionary, the
 `baz` method should be applicable.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7123#comment:1>
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