[GHC] #14760: Error reporting on obsolete file

[GHC]

#14760: Error reporting on obsolete file
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           Reporter:  hsyl20         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           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:
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 I have just had this warning:
 {{{
 Whatever.hs:41:1: warning: [-Wunused-imports]
     The import of ‘Data.Map’ is redundant
       except perhaps to import instances from ‘Data.Map’
     To import instances alone, use: import Data.Map()
    |
 41 | import Data.Foldable
    | ^^^^^^^^^^^^^^^^^^^^^
 }}}

 This happens because the source file has been modified during the
 compilation: GHC shows an excerpt of the new file to report an error in
 the old file. Could we cache the old file somehow during the compilation
 to avoid this discrepancy?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14760>
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