[GHC] #9918: GHC chooses an instance between two overlapping, but cannot resolve a clause within the similar closed type family

[GHC]

#9918: GHC chooses an instance between two overlapping, but cannot resolve a
clause within the similar closed type family
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        Reporter:  qnikst            |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 I feel stuck on this.  I can see Oleg's goal, but I don't want to require
 him to use a potentially-unsound route to gain it.  It's tantalising that
 overlapping type classes achieve the same end without risking unsoundness.
 Surely we must be able to replicate this in type families, somehow?

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9918#comment:17>
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