[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):

 Replying to [comment:3 simonpj]:
 > I'm puzzled though.  The `{-# COMPLETE MkT1 #-}` pragma seems like a
 manifest lie.  It claims that any matching on `MkT2` is inaccessible
 doesn't it?

 `COMPLETE` pragmas are allowed to be lies. Perhaps values built from
 `MkT1` are never visible outside this module.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14253#comment:4>
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