[GHC] #15139: EmptyCase produces incorrect SrcSpans

10 May 2018

      #15139: EmptyCase produces incorrect SrcSpans
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.2
  (Parser)                           |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Poor/confusing
  Unknown/Multiple                   |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 If you compile the following program, you'll get a warning for `f` with an
 improperly placed `SrcSpan`:

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

 import Data.Type.Equality

 can'tHappen :: Int :~: Bool
 can'tHappen = undefined

 f, g :: Bool -> Bool
 f True = case can'tHappen of {}
 g True = case () of () -> True
 }}}
 {{{
 $ /opt/ghc/8.4.2/bin/ghci Bug.hs
 GHCi, version 8.4.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:12:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘f’: Patterns not matched: False
    |
 12 | f True = case can'tHappen of {}
    | ^^^^^^^^^^^^^

 Bug.hs:13:1: warning: [-Wincomplete-patterns]
     Pattern match(es) are non-exhaustive
     In an equation for ‘g’: Patterns not matched: False
    |
 13 | g True = case () of () -> True
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 Ok, one module loaded.
 }}}

 Compare that to the warning for `g`, whose `SrcSpan` spans the entire
 clause, as expected.

 The bug lies in the parser—patch incoming.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15139
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