[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:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:  Design/Warnings   |
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Comment (by hvr):

 PS: there some circular implementation details here I'm deliberately
 ignoring; I'm aware this may require some special handling for this new
 flag, rather than being handled uniformly like the other flags. I'm more
 inclined to have a uniform CLI appearance to the user than a uniform
 implementation...

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11429#comment:1>
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