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

[GHC]

#11429: Make unrecognised `-W` flags a warning rather than an error
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        Reporter:  hvr               |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D1830,
       Wiki Page:  Design/Warnings   |  Phab:D1942
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Changes (by hvr):

 * owner:  bgamari =>
 * status:  closed => new
 * differential:  Phab:D1830 => Phab:D1830, Phab:D1942
 * resolution:  fixed =>


Comment:

 See Phab:D1492 for extending this warning to `-f(no-)warn-*` flags as well

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11429#comment:7>
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