[GHC] #9888: Inconsistent type family resolution

[GHC]

#9888: Inconsistent type family resolution
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              Reporter:  crockeea    |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:  duplicate   |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by crockeea):

 I get the same error using `(:==$)`:

 {{{#!hs
 type family CommonElt xs where
   CommonElt (x ': xs) =
     EqResult (And (Map ((:==$) $ x) xs)) x
 }}}

 but I think that is related to a `singletons`
 [https://github.com/goldfirere/singletons/issues/100 ticket]

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9888#comment:2>
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