[Bug c/108796] Can't intermix C2x and GNU style attributes

["aaron at aaronballman dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108796

--- Comment #9 from Aaron Ballman <aaron at aaronballman dot com> ---
> GNU attributes are declaration specifiers *in the previous examples given 
> here*, not necessarily in all other cases.

Thanks for clarifying!

> (There is then logic in GCC to handle __attribute__ that, according to the 
> syntax, should appertain to a particular entity, so that it's instead 
> applied to some other related entity; for example, moving an attribute 
> from a declaration to its type.  This is deliberately *not* done for [[]] 
> attribute syntax; those attributes are expected to be written in a correct 
> location for the entity they appertain to.)

This touches on why I came to the decision I did in Clang. What `__attribute__`
will apply to is sometimes inscrutable and users are (perhaps) used to it
sliding around to whatever works. As you point out, `[[]]` doesn't have the
same behavior; it has strict appertainment. Because `__attribute__` doesn't
have strict appertainment, it did not seem like an issue for it to continue to
shift around to whatever makes sense. Thus `[[]]` will apply to what the
standard says it applies to, and `__attribute__` applies to whatever it should
apply to based on the attribute names in the specifier, but users don't have to
know whether they need to write `[[]] __attribute__(())` vs `__attribute__(())
[[]]`. (Clang also supports `__declspec`, so there are more combinations to
worry about sometimes.)

It really boils down to whether `__attribute__` is fundamentally a different
"thing" than `[[]]` and I couldn't convince myself they were different. The
result is, when the grammar allows consecutive attribute syntaxes, we parse all
allowed syntaxes in a loop so users can write them in an arbitrary order.