https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53548
--- Comment #7 from Kees Cook <kees at outflux dot net> --- There is still no way to use C99 flexible arrays in unions (or alone in structs) without syntactic obfuscation. The extension that already allows 0-sized arrays in unions should be extended to cover C99 arrays. This is especially important for projects migrating away from the various deprecated "fake" flexible array members to C99 flex array members, as they continue to depend on both union membership and single-member structs (i.e. the Linux kernel has lots of these, some even in UAPI). Please reopen this bug. :) Clang is also preparing to fix this issue.
