[GHC] #15753: Inconsistent pattern-match warnings when using guards versus case expressions

[GHC]

#15753: Inconsistent pattern-match warnings when using guards versus case
expressions
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.1
      Resolution:                    |             Keywords:
                                     |  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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Even simpler:

 {{{#!hs
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE TypeOperators #-}
 {-# OPTIONS_GHC -Wincomplete-patterns #-}
 module Bug where

 import Data.Type.Equality

 data G a where
   GInt  :: G Int
   GBool :: G Bool

 ex1, ex2, ex3
   :: a :~: Int
   -> G a
   -> ()

 ex1 Refl g
   | GInt <- id g
   = ()

 ex2 Refl g
   | GInt <- g
   = ()

 ex3 Refl g
   = case id g of
       GInt -> ()
 }}}
 {{{
 $ /opt/ghc/8.6.1/bin/ghci Bug.hs
 GHCi, version 8.6.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/ryanglscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:17:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘ex1’: Patterns not matched: Refl _
    |
 17 | ex1 Refl g
    | ^^^^^^^^^^...
 }}}

 This time, I've included three examples. In particular, note that `ex1`
 and `ex2` are //extremely// similar—the only difference is the use of
 `id`!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15753#comment:2>
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