[GHC] #8506: misleading error message for duplicate type class instances

[GHC]

#8506: misleading error message for duplicate type class instances
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        Reporter:  carter                   |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.6.3
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by carter):

 GHC should give an erro about multiple class instances, but I guess the
 renamer phase hits the constants in the instance heads before that
 multiple declaration conflict would happen.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8506#comment:1>
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