[GHC] #11429: Make unrecognised `-W` flags a warning rather than an error

[GHC]

#11429: Make unrecognised `-W` flags a warning rather than an error
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  bgamari
            Type:  feature request   |               Status:  patch
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1830
       Wiki Page:  Design/Warnings   |
-------------------------------------+-------------------------------------
Changes (by bgamari):

 * status:  new => patch
 * differential:   => Phab:D1830


Comment:

 Implementation in Phab:D1830.

 Note that this flag is order dependent. That is,
 {{{
 ghc -Wfoobar -Wno-unrecognised-warning-flag
 }}}
 will throw a warning but
 {{{
 ghc -Wno-unrecognised-warning-flag -Wfoobar
 }}}
 will not. This, however, is already true of existing flags (e.g. see
 #10560).

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11429#comment:3>
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