[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 simonpj):

 OK, but then lies might give rise to error messages or warnings that lie,
 mightn't they.  As here.

 We need a specification of the expected behaviour of lying COMPLETE
 pragmas before we can discuss what programs like these "should" do.  Or we
 could say "if the pragma lies, the compiler can do what it likes".

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14253#comment:5>
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