[GHC] #2356: GHC accepts multiple instances for the same type in different modules

[GHC]

#2356: GHC accepts multiple instances for the same type in different modules
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        Reporter:  claus             |            Owner:
            Type:  bug               |           Status:  new
        Priority:  low               |        Milestone:  ⊥
       Component:  Compiler          |          Version:  6.8.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #5316
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Comment (by rwbarton):

 You can get this kind of instance incoherence without orphan instances or
 any extensions beyond `FlexibleInstances`. See
 https://gist.github.com/rwbarton/dd8e51dce2a262d17a80.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2356#comment:16>
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