[Bug c++/79301] With -Werror=pedantic outside C++17 mode, __has_cpp_attribute(fallthrough) is nonzero but [[fallthrough]] fails

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79301

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 40713
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40713&action=edit
gcc7-pr79301.patch

If that is what we want, I think this untested patch should implement it.