[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:"b9c697eefd34fcba3ab9c1f831baf7f651ad7503/ghc"
 b9c697ee/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="b9c697eefd34fcba3ab9c1f831baf7f651ad7503"
 Print which flag controls emitted desugaring warnings

 This is extends bb5afd3c274011c5ea302210b4c290ec1f83209c to cover
 warnings emitted during the desugaring phase.

 This implements another part of #10752

 Reviewed-by: quchen, bgamari

 Differential Revision: https://phabricator.haskell.org/D1954
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10752#comment:16>
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