[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):

 Yes, but the library-writer could just use a newtype to get this
 functionality. For example, if the writer of `Set` wants coercions
 internally, they use `SetInternal` with no role annotations and then `Set`
 with a role annotation. It's a little bit of (compile-time-only) overhead,
 but it should be usable.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9117#comment:13>
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