https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127311
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> --- Well, C++ doesn't have flexible array members at all (it is only a GNU extension), so using -fstrict-flex-arrays=3 for C++ doesn't make much sense, standard C++ code has to use workarounds for it (or not the trailing array idioms at all).
