[GHC] #9117: Coercible constraint solver misses one

[GHC]

#9117: Coercible constraint solver misses one
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        Reporter:  goldfire          |            Owner:  nomeata
            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 goldfire):

 I suppose it would. But, if we decide not to let my example in comment:7
 work, we need to articulate this clearly somewhere. According to all the
 "rules" around `Coercible`, it would seem that the code in comment:7
 should work and may surprise folks when it doesn't.

 I have to say I don't love the idea of not handling comment:7, but if we
 can't find a way to do it, it's not the end of the world. Would it make
 sense to detect the recursion and then switch techniques? Seems ad-hoc.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:23>
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