[GHC] #12099: ghc --show-options lists invalid flags

[GHC]

#12099: ghc --show-options lists invalid flags
-------------------------------------+-------------------------------------
           Reporter:  DanielG        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While porting ghc-mod to GHC 8 I noticed that all `-fwarn` flags are
 missing from `flagsForCompletion` instead only `-fwarn-` and `-fno-warn-`
 appear. This also shows up in the front-end when running `ghc --show-
 options`.

 Looking at the code this turned out to be because the old `-fwarn` flags
 were marked as hidden using `hideFlag` and hidden flags are explicitly
 filtered out in `flagsForCompletion` but for some reason the `-fwarn-` and
 `-fno-warn-` prefixes still show up in the list.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12099>
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