[GHC] #13965: COMPLETE sets nerf redundant pattern-match warnings

12 Jul 2017


      #13965: COMPLETE sets nerf redundant pattern-match warnings
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc2
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms,                   |
  PatternMatchWarnings               |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If I give redundant patterns for a datatype with a `COMPLETE` set, then
 GHC doesn't warn me at all. Here's an example:

 {{{#!hs
 {-# LANGUAGE PatternSynonyms #-}
 {-# LANGUAGE ViewPatterns #-}
 module Bug (Boolean(F, TooGoodToBeTrue), catchAll) 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 F               = 2
 }}}

 GHC thinks this is fine and dandy:

 {{{
 $ /opt/ghc/8.2.1/bin/ghci Bug.hs -Wall
 GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )
 Ok, 1 module loaded.
 }}}

 This problem also persists across modules:

 {{{#!hs
 module Foo where

 import Bug

 catchAll2 :: Boolean -> Int
 catchAll2 F               = 0
 catchAll2 TooGoodToBeTrue = 1
 catchAll2 F               = 2
 }}}
 {{{
 $ /opt/ghc/8.2.1/bin/ghci Foo.hs -Wall
 GHCi, version 8.2.0.20170704: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 2] Compiling Bug              ( Bug.hs, interpreted )
 [2 of 2] Compiling Foo              ( Foo.hs, interpreted )
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13965
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler