[GHC] #9258: Type inference fails with closed type families

[GHC]

#9258: Type inference fails with closed type families
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        Reporter:  augustss          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by jstolarek):

 I'm not an expert here, but it looks like you expect GHC to infer the
 arguments of s type family based on its result. I believe that's not
 possible in general and my guess is that GHC doesn't even try, even for a
 trivial case such as yours.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9258#comment:1>
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