[GHC] #15886: Spurious warning about incomplete pattern with PatternSynonyms

[GHC]

#15886: Spurious warning about incomplete pattern with PatternSynonyms
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        Reporter:  selinger          |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.1
      Resolution:  invalid           |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  Incorrect         |  (amd64)
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by selinger):

 Interesting. Thanks for pointing this out. I assumed (wrongly, it turns
 out) that since pattern synonyms are just syntactic sugar, the checker
 could desugar it for the purposes of coverage checking. Are there good
 reasons for "the checker is very naive"?

 For sure, there are good uses cases for COMPLETE pragmas, because there
 are cases (even without using pattern synonyms) that's aren't readily
 decidable. I'm just surprised that the checker can't solve obvious cases.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15886#comment:2>
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