[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
-------------------------------------+-------------------------------------
        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:  #11984, #14098    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"0e60cc1825aace414597b644731c30269994f7fb/ghc" 0e60cc1/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="0e60cc1825aace414597b644731c30269994f7fb"
 Document how GHC disambiguates between multiple COMPLETE sets

 Summary:
 Up until now, the knowledge of how GHC chooses which
 `COMPLETE` set to use in the presence of multiple applicable
 `COMPLETE` sets for a single data type constructor was only
 documented in the GHC wiki. But this really should be advertised to
 anyone who uses `COMPLETE` pragmas heavily, so per SPJ's advice in
 https://ghc.haskell.org/trac/ghc/ticket/14253#comment:10, this adds
 this wisdom to the GHC users' guide.

 Test Plan: Read it

 Reviewers: austin, bgamari

 Subscribers: mpickering, rwbarton, thomie

 GHC Trac Issues: #14253

 Differential Revision: https://phabricator.haskell.org/D4005
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14253#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler