[GHC] #3440: Improve error message for GADT failures

[GHC]

#3440: Improve error message for GADT failures
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  Reporter:  simonpj                  |          Owner:                  
      Type:  bug                      |         Status:  new             
  Priority:  normal                   |      Milestone:  6.12 branch     
 Component:  Compiler (Type checker)  |        Version:  6.10.4          
Resolution:                           |       Keywords:                  
Difficulty:  Unknown                  |             Os:  Unknown/Multiple
  Testcase:                           |   Architecture:  Unknown/Multiple
   Failure:  None/Unknown             |  
--------------------------------------+-------------------------------------
Changes (by igloo):

  * failure:  => None/Unknown
  * milestone:  6.12.1 => 6.12 branch

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3440#comment:1>
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