[GHC] #12752: GHC should be able to warn on unsafe CPP usage

[GHC]

#12752: GHC should be able to warn on unsafe CPP usage
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        Reporter:  erikd             |                Owner:  erikd
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  phab:D2626
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"3cb32d8b0b51c548ab424139c66cce6b37a2ab1b/ghc" 3cb32d8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="3cb32d8b0b51c548ab424139c66cce6b37a2ab1b"
 Add -Wcpp-undef warning flag

 When enabled, this new warning flag passes `-Wundef` to the C
 pre-processor which causes the pre-processor to warn on uses of
 the `#if` directive on undefined identifiers.

 It is not currently enabled in any of the standard warning groups.

 Test Plan: Make sure the two tests pass on all major platforms.

 Reviewers: hvr, carter, Phyx, bgamari, austin

 Reviewed By: Phyx

 Subscribers: thomie

 Differential Revision: https://phabricator.haskell.org/D2626

 GHC Trac Issues: #12752
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12752#comment:5>
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