[GHC] #13717: Pattern synonym exhaustiveness checks don't play well with EmptyCase

[GHC]

#13717: Pattern synonym exhaustiveness checks don't play well with EmptyCase
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1-rc1
      Resolution:                    |             Keywords:
                                     |  PatternSynonyms,
                                     |  PatternMatchWarnings
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  error/warning at compile-time      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 dfeuer, thanks. Now I see why my example didn't trigger any warnings: the
 definition of the `T` type itself (i.e., the fact that it was a GADT) was
 what ruled out the possibility of matching on any of its constructors in
 `tCharAbsurd`. On the other hand, `Fin` does not have this property, and
 you're relying on the types of the conlikes in its `COMPLETE` set to rule
 out the possibility of matching anything.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13717#comment:6>
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