[Bug c++/109877] Support for clang-style attributes is needed to parse Darwin SDK headers properly

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109877

--- Comment #5 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #4)
> (In reply to Andrew Pinski from comment #2)
> > This style of attributes is bad. Because the GNU style attribute is just
> > token(expression,expression,expression) it seems odd that they added these
> > kind of attributes without thinking maybe it would be rejected somewhere
> > else.
> 
> To be precise, some attributes have the first argument an identifier rather
> than expression (e.g. mode/format/cleanup/access attribute).
> So, the macOS as first argument is acceptable, but not the second one.

As far as I can tell (and what my current implementation does) is to say that
everything in the inner parentheses cam be lexed/parsed in a manner specific to
the attribute ID.

so in:
 availability(zzzz) the zzzz is parsed specifically to "availability"
and presumably:
 another_attr(yyyy) could have a different set of rules.

As noted, we [at least, all I have spoken to] agree that this is bad (in
respect of deviating from GNU style without any other mechanism to
differentiate).  However, once stuff is in the wild....