[GHC] #8581: Add support for explicitly-bidirectional pattern synonyms

[GHC]

#8581: Add support for explicitly-bidirectional pattern synonyms
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        Reporter:  cactus            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |  PatternSynonyms
 Type of failure:  None/Unknown      |            Architecture:
      Blocked By:  5144              |  Unknown/Multiple
 Related Tickets:                    |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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Comment (by rwbarton):

 dmcclean, I was able to compile your program with ghc-7.10.0.20150123,
 though I had to add the RankNTypes language extension as well; can you
 double check?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:24>
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