Review accepted with slight revision
On February 1, 2018 the Core Team decided to accept this proposal with slight revision over the original proposal.
The only revision over the original proposal is to change the syntax to use #warning(<Message>) instead of #warning <Messsage>. This fits well with most of Swift's existing compiler directives, and was strongly supported in the review discussion.
The review discussion also covered a variety of possible extensions or variants to this proposal, including support for using #warning as an expression instead of a line directive and support for runtime issues. The Core Team decided that while these directions are interesting and worth exploring, they are complementary to the core functionality serviced by this proposal. Further, keeping #warning as a line directive allows it to be used in a wide variety of contexts, and serves a different need than using it as a placeholder expression.