[GHC] #16129: Incorrect non-exhaustive pattern warning with PatternSynonyms, ViewPatterns and TypeFamilies

4 Jan 2019

      #16129: Incorrect non-exhaustive pattern warning with PatternSynonyms, ViewPatterns
and TypeFamilies
-------------------------------------+-------------------------------------
           Reporter:  sjakobi        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           Keywords:                 |  Operating System:  Unknown/Multiple
  PatternMatchWarnings               |
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# language PatternSynonyms #-}
 {-# language ViewPatterns #-}
 {-# language TypeFamilies #-}

 module Bug where

 newtype GenLocated e = L e

 newtype WithSourceText a = WithSourceText a

 type StringLiteral = WithSourceText String

 {-# COMPLETE StringLiteral #-}
 pattern StringLiteral sl_s = WithSourceText sl_s

 type family SrcSpanLess a

 class HasSrcSpan a where
   decomposeSrcSpan :: a -> GenLocated (SrcSpanLess a)

 type instance SrcSpanLess (GenLocated e) = e

 instance HasSrcSpan (GenLocated a) where
   decomposeSrcSpan = id

 bug :: GenLocated StringLiteral -> String
 bug (decomposeSrcSpan->L (StringLiteral s)) = s

 workaround :: GenLocated StringLiteral -> String
 workaround (decomposeSrcSpan->L sl) = s
   where StringLiteral s = sl
 }}}

 {{{
 $ ghci -Wincomplete-patterns Bug.hs
 GHCi, version 8.7.20190103: https://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/simon/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:27:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘bug’: Patterns not matched: _
    |
 27 | bug (decomposeSrcSpan->L (StringLiteral s)) = s
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Ok, one module loaded.
 }}}

 I've been able to reproduce the issue with all versions of GHC >= 7.8.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16129
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler