[GHC] #14253: Pattern match checker mistakenly concludes pattern match on pattern synonym is unreachable

[GHC]

#14253: Pattern match checker mistakenly concludes pattern match on pattern synonym
is unreachable
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Ah, I think Ryan's simplification goes a bit too far, perhaps. I think
 you'll find Ben's original version convincing.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14253#comment:6>
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