[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        Reporter:  nomeata           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.1
  checker)                           |             Keywords:
      Resolution:                    |  PatternSynonyms
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 You can define `SnocNil` via the same view (e.g. `viewr -> EmptyR`), then
 from my last bullet point GHC will see that the patterns are exhaustive.
 There will certainly be other holes in the scheme though.

 Even if we had `COMPLETE_PATTERNS`, I would still expect GHC to be able to
 handle expanding pattern synonyms to their definitions and collecting view
 patterns with the same view without any help from `COMPLETE_PATTERNS`
 pragmas; and then I don't see much additional value in the pragma to
 justify its implementation given its limitations. I could get behind a
 more comprehensive solution, though I don't know what it would look like.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:33>
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