[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):

 I tried to reproduce this error, but I couldn't. I don't know what
 implementations for `FZ` and `FS` you had in mind, so I tried whipping up
 my own example from scratch:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE EmptyCase #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PatternSynonyms #-}
 {-# OPTIONS_GHC -Wall #-}
 module T (T(TInt, TBool), tCharAbsurd1) where

 import Data.Void

 data T a where
   MkTInt  :: T Int
   MkTBool :: T Bool

 pattern TInt :: () => n ~ Int => T n
 pattern TInt = MkTInt

 pattern TBool :: () => n ~ Bool => T n
 pattern TBool = MkTBool
 {-# COMPLETE TInt, TBool #-}

 tCharAbsurd1 :: T Char -> Void
 tCharAbsurd1 x = case x of {}
 }}}

 This, however, compiles without any warnings. I even tried using `T` in
 another module:

 {{{#!hs
 {-# LANGUAGE EmptyCase #-}
 {-# OPTIONS_GHC -Wall #-}
 module Bug where

 import Data.Void
 import T

 tCharAbsurd2 :: T Char -> Void
 tCharAbsurd2 x = case x of {}
 }}}

 This too compiles without warning:

 {{{
 $ ghc/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.3.20170516: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 2] Compiling T                ( T.hs, interpreted )
 [2 of 2] Compiling Bug              ( Bug.hs, interpreted )
 Ok, modules loaded: Bug, T.
 }}}

 How did you get this supposed pattern coverage warning?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13717#comment:3>
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