[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
-------------------------------------+-------------------------------------
       Reporter:  qnikst             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  None/Unknown
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 We hoped that closed type families could replace overlapping
 instances. It is not currently the case: GHC can resolve overlapping
 instances but cannot resolve the similar closed type family.

 I'm attaching a minimized example of program where it can
 be observed. This is reproducible on ghc-7.8.3 and HEAD.

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