[GHC] #15014: Exhaustivity check should suggest when COMPLETE could be helpful

[GHC]

#15014: Exhaustivity check should suggest when COMPLETE could be helpful
-------------------------------------+-------------------------------------
           Reporter:  ezyang         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.3
          Component:  Compiler       |           Version:  8.5
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternSynonyms,                   |
  PatternMatchWarnings               |
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{
 MacBook-Pro-97:ghc ezyang$ cat A.hs
 {-# LANGUAGE PatternSynonyms #-}
 module A where

 pattern F :: a -> b -> (a, b)
 pattern F x y = (x, y)

 g :: (a, b) -> (a, b)
 g (F x y) = (x, y)
 MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 -c A.hs -Wall -fforce-
 recomp

 A.hs:8:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’: Patterns not matched: _
   |
 8 | g (F x y) = (x, y)
   | ^^^^^^^^^^^^^^^^^^
 MacBook-Pro-97:ghc ezyang$ inplace/bin/ghc-stage2 --version
 The Glorious Glasgow Haskell Compilation System, version 8.5.20180304
 }}}

 Any time the exhaustiveness checker throws up its hands and says that `_`
 is not matched, we ought to give a hint that this may have occurred due to
 pattern synonyms, and that the author of the pattern synonyms can help out
 by specifying a COMPLETE pragma.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15014>
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