[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" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #8 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jason Merrill from comment #7)
> (In reply to Jakub Jelinek from comment #4)
> > That said, I think e.g. for maybe_unused or nodiscard attributes we don't
> > complain with -pedantic about those attributes used in C++14 code, so either
> > we should do that as well, or we shouldn't do that for fallthrough either.
> > Jason?
> 
> We certainly don't want to both have __has_cpp_attribute(fallthrough)
> non-zero and also complain about its usage.  My inclination would be to
> remove the diagnostic.

Also for deprecated attribute and C++11?
      else if (is_attribute_p ("deprecated", attr_id))
        {
          if (cxx_dialect == cxx11)
            pedwarn (token->location, OPT_Wpedantic,
                     "%<deprecated%> is a C++14 feature;"
                     " use %<gnu::deprecated%>");
          TREE_PURPOSE (TREE_PURPOSE (attribute)) = get_identifier ("gnu");
        }
      /* C++17 fallthrough attribute is equivalent to GNU's.  */
      else if (is_attribute_p ("fallthrough", attr_id))
        {
          if (cxx_dialect < cxx1z)
            pedwarn (token->location, OPT_Wpedantic,
                     "%<fallthrough%> is a C++17 feature;"
                     " use %<gnu::fallthrough%>");
          TREE_PURPOSE (TREE_PURPOSE (attribute)) = get_identifier ("gnu");
        }
Even for that one can test #if __has_cpp_attribute(deprecated) and
[[deprecated]] still won't work if -std=c++11 -pedantic-errors.