[GHC] #2851: Improve error message for failed deriving

[GHC]

#2851: Improve error message for failed deriving
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    Reporter:  guest                       |        Owner:  igloo           
        Type:  merge                       |       Status:  new             
    Priority:  normal                      |    Milestone:                  
   Component:  Compiler (Type checker)     |      Version:  6.10.1          
    Severity:  minor                       |   Resolution:                  
    Keywords:                              |   Difficulty:  Unknown         
    Testcase:  deriving/should_fail/T2851  |           Os:  Unknown/Multiple
Architecture:  Unknown/Multiple            |  
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Comment (by simonpj):

 PS: I did a bit more, because the previous patch suggested the same fix
 for overlapping instances, which is misleading
 {{{
 Wed Dec 31 06:35:21 PST 2008  simonpj at microsoft.com
   * Improve error reporting for 'deriving'
 }}}
 Simon

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2851#comment:8>
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