[GHC] #12056: Too aggressive `-w` option

[GHC]

#12056: Too aggressive `-w` option
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        Reporter:  asr               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11429, #11789    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by wozgonon):

 Error appears to be in parsing of flags before DynFlags.
 See comment at end of ghc/compiler/main/CmdLineParser.hs

 {{{
 errorsToGhcException errs =
     UsageError $ intercalate "\n" $ [ l ++ ": " ++ e | (l, e) <- errs ]

 {- Note [Handling errors when parsing commandline flags]

 Parsing of static and mode flags happens before any session is started,
 i.e.,
 before the first call to 'GHC.withGhc'. Therefore, to report errors for
 invalid usage of these two types of flags, we can not call any function
 that
 needs DynFlags, as there are no DynFlags available yet
 (unsafeGlobalDynFlags
 is not set either). So we always print "on the commandline" as the
 location,
 which is true except for Api users, which is probably ok.

 When reporting errors for invalid usage of dynamic flags we /can/ make use
 of
 DynFlags, and we do so explicitly in DynFlags.parseDynamicFlagsFull.

 Before, we called unsafeGlobalDynFlags when an invalid (combination of)
 flag(s) was given on the commandline, resulting in panics (#9963).
 -}
 }}}


 The actual error displayed seems to originate from
 ghc/compiler/main/StaticFlags.hs which calls the above
 'errorsToGhcException' in CmdLineParser.hs

 {{{
 StaticFlags.hs:86:      errorsToGhcException . map (("on the commandline",
 ) . unLoc) $ errs
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12056#comment:4>
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