[GHC] #15917: GHC seems to re-read source of module to produce fancy error msgs

[GHC]

#15917: GHC seems to re-read source of module to produce fancy error msgs
-------------------------------------+-------------------------------------
           Reporter:  Heimdell       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  lowest         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.2.2
           Keywords:  re-reading     |  Operating System:  Unknown/Multiple
       Architecture:  x86_64         |   Type of failure:  Poor/confusing
  (amd64)                            |  error message
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I had a module with unused name `txFees` in it. I started compilation and
 in the same time edited code by renaming `txFees` -> `_txFees`.

 I got the following message:

 {{{
     warning: [-Wunused-matches]
         Defined but not used: ‘txFees’
        |
     40 |             } _txFees
        |               ^^^^^^

     <no location info>: error:
     Failing due to -Werror.
 }}}

 The screenshot is in the attachment. Notice that message part is about old
 code (`txFees`), and preview is about new code (`_txFees`).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15917>
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