[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:  patch
        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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Changes (by cactus):

 * status:  new => patch


Comment:

 Pushed `576f461` to `wip/pattern-synonyms`, please review for `HEAD`. It
 validates and has a test case for bidirectional pattern synonyms.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8581#comment:5>
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