[GHC] #13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant pattern-match warnings

2 Mar 2017

      #13363: Wildcarn patterns and COMPLETE sets can lead to misleading redundant
pattern-match warnings
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms                    |
       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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 Consider this program:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ViewPatterns #-}
 {-# OPTIONS_GHC -Wincomplete-patterns #-}
 module Bug where

 data Boolean = F | T
   deriving Eq

 pattern TooGoodToBeTrue :: Boolean
 pattern TooGoodToBeTrue <- ((== T) -> True)
   where
     TooGoodToBeTrue = T
 {-# COMPLETE F, TooGoodToBeTrue #-}

 catchAll :: Boolean -> Int
 catchAll F               = 0
 catchAll TooGoodToBeTrue = 1
 }}}

 This compiles with no warnings with `-Wall`. But if you tweak `catchAll`
 to add a catch-all case at the end:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ViewPatterns #-}
 {-# OPTIONS_GHC -Wincomplete-patterns #-}
 module Bug where

 data Boolean = F | T
   deriving Eq

 pattern TooGoodToBeTrue :: Boolean
 pattern TooGoodToBeTrue <- ((== T) -> True)
   where
     TooGoodToBeTrue = T
 {-# COMPLETE F, TooGoodToBeTrue #-}

 catchAll :: Boolean -> Int
 catchAll F               = 0
 catchAll TooGoodToBeTrue = 1
 catchAll _               = error "impossible"
 }}}

 Then if you compile it with `-Wall`, you'll get a very misleading warning:

 {{{
 $ ~/Software/ghc2/inplace/bin/ghc-stage2 --interactive Bug.hs -Wall
 GHCi, version 8.1.20170228: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:17:1: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In an equation for ‘catchAll’: catchAll TooGoodToBeTrue = ...
    |
 17 | catchAll TooGoodToBeTrue = 1
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I would have expected the warning to be on the `catchAll _               =
 error "impossible"` case!

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13363
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