[GHC] #10752: Print which warning-flag controls/enabled an emitted warning

[GHC]

#10752: Print which warning-flag controls/enabled an emitted warning
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        Reporter:  hvr               |                Owner:  barrucadu
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       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:D1943
       Wiki Page:  Design/Warnings   |
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Comment (by Herbert Valerio Riedel <hvr@…>):

 In [changeset:"46f3775c683faeb710c9dc22f360f39334947d73/ghc"
 46f3775c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="46f3775c683faeb710c9dc22f360f39334947d73"
 Default to -fno-show-warning-groups (re #10752)

 As `-fno-show-warning-groups` shows associated warning groups regardless
 of whether the respective warning group flag as been passed on the CLI,
 the warning-group information may be confusing to users.

 At this point, `-fshow-warning-groups` is useful mostly to GHC
 developers and possibly GHC users who want to see which warning groups
 an emitted warning is part of. (Btw, this is particularly interesting in
 combination with `-Weverything` which enables *every* warning flag known
 to GHC.)

 Consequently, starting with this commit, one has to opt-in via
 `-fshow-warning-groups` for GHC to show warning groups.

 In order to reduce the testsuite delta in this commit, the
 `-fshow-warning-groups` flag has been added to TEST_HC_OPTS.
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10752#comment:21>
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