[GHC] #7989: "No constructor has all these fields" message can be improved

[GHC]

#7989: "No constructor has all these fields" message can be improved
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    Reporter:  akio              |       Owner:                  
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:                  
   Component:  Compiler          |     Version:  7.7             
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  Other           
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by monoidal):

 Do you handle this situation?

 {{{
 data T = A { x,y :: Int} | B { y,z :: Int } | C { z,x :: Int}
 f a = a { x=0, y=0, z=0 }
 }}}

 Error message here cannot mention only two fields.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7989#comment:5>
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