https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817
--- Comment #7 from jim at meyering dot net --- Thanks for investigating. I already pushed a workaround for gnulib's vasnprintf problem (http://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=e63f5eb55570a1ea3e51ce47df33689785e085c1). I may just leave that there. The alternative of using an empty "__attribute__((__fallthrough__));" statement would require a macro expanding to that or to nothing.
