I haven't been following the discussion very closely so I may have missed that what I'm about to suggest has been discussed and rejected for some valid reason, but if not let me try.
It seems to me that the ultimate, long term goal should be to have actively maintained code bases gradually migrate away from the various fallthrough comments and to the new attribute. Under that premise, I think introducing a warning that's on the permissive end of the spectrum (say outside of -Wall, or even outside of -Wextra, and/or disabling the checker at the first sight of a comment) would obviate the concern of needlessly breaking working code and let users start adopting the warning on their own schedules. The next major release of GCC after 7 could increase the sensitivity of the warning (e.g., by adding it -Wextra, and/or checking for the words fall though in the comments, etc.), and the next one could make it even more strict. With GCC's one year release cycle this approach would give even users who adopt the latest compiler two to three years to make the transition. Martin