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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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