[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:  cactus
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:  5144
        Blocking:                    |  Related Tickets:
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Comment (by dfranke):

 `_` in expressions is already in use for typed holes
 (http://www.haskell.org/ghc/docs/latest/html/users_guide/typed-
 holes.html).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:3>
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