[GHC] #8779: Exhaustiveness checks for pattern synonyms

[GHC]

#8779: Exhaustiveness checks for pattern synonyms
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        Reporter:  nomeata           |                Owner:  mpickering
            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):  Phab:D2669
       Wiki Page:                    |
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Comment (by rwbarton):

 Ah okay. But that doesn't require knowing anything at all about `f`. It
 just requires that you used the same `f` in all the equations, and then
 chose a covering family `p1, ..., pn` of patterns for its output.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:49>
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