[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:  closed
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:
      Resolution:  fixed             |             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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Comment (by thomie):

 Is it by design that `-Wno-unrecognised-warning-flags` should come first
 on the command line?

 {{{
 $ ghc-8.0.1 Test.hs -Wno-unrecognised-warning-flags -Wfoo
 # no warning, as expected

 $ ghc-8.0.1 Test.hs -Wfoo -Wno-unrecognised-warning-flags
 on the commandline: warning: unrecognised warning flag: -Wfoo
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11429#comment:11>
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