[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 hvr):

 I just stumbled over an annoying case due to our new `ErrorCall` in GHC
 8.0:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 module Patterns where

 -- import Control.Exception (ErrorCall(..))
 data ErrorCall = ErrorCallWithLocation String String deriving (Eq, Ord)

 pattern ErrorCall :: String -> ErrorCall
 pattern ErrorCall err <- ErrorCallWithLocation err _
   where ErrorCall err =  ErrorCallWithLocation err ""

 getMsg :: ErrorCall -> String
 getMsg (ErrorCall y) = y

 getMsg' :: ErrorCall -> String
 getMsg' (ErrorCallWithLocation y _) = y
 }}}

 With that, GHC HEAD currently warns

 {{{
 patterns.hs:12:1: warning:
     Pattern match(es) are non-exhaustive
     In an equation for ‘getMsg’: Patterns not matched: _
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8779#comment:12>
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