[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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Description changed by dfeuer:

@@ -8,1 +8,1 @@
- pattern FZ :: () => forall m. n ~ 'S m => Fin n
+ pattern FZ :: () => n ~ 'S m => Fin n

New description:

 In #8779, mpickering added a `COMPLETE` pragma to allow exhaustiveness
 checking for matches involving pattern synonyms. Unfortunately, there is
 still one piece missing: the interaction with `EmptyCase`. Suppose I write

 {{{#!hs
 newtype Fin (n :: Nat) = Fin Natural  -- constructor not exported

 pattern FZ :: () => n ~ 'S m => Fin n
 pattern FS :: () => n ~ 'S m => Fin m -> Fin n
 {-# COMPLETE FZ, FS #-}
 }}}

 I would very much like to be able to write

 {{{#!hs
 finZAbsurd :: Fin 'Z -> Void
 finZAbsurd x = case x of
 }}}

 but this gives me a pattern coverage warning! That's certainly not what we
 want. I believe what we actually want is this:

   When checking empty case, check that ''at least one'' complete pattern
 set is impossible.

 In this case, it would see two complete pattern sets: the built-in `{Fin}`
 and the user-decreed `{FZ, FS}`. Since neither `FZ` nor `FS` have matching
 types, we should conclude that the empty case is fine.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13717#comment:1>
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