I refer you to the various issues I have raised with attributes proposals in WG14 in the past, some at least of which have also been forwarded to WG21. The committees have generally chosen not to listen to compatibility concerns, except insofar as WG14 ended up not adopting general attribute syntax at all but instead adding new keywords for some particular uses of attributes.
I also quote here something I sent off-reflector to Herb Sutter (Microsoft) last September (see in particular the first paragraph regarding treating [[]] as having clearly separate rules from __attribute__): [...] I'm also concerned about things for users. If I implement the proposals in GCC, compatibility with existing code would inevitably mean that __attribute__ follows existing rules for how it binds to syntax constructs, and [[]] follows the different C1x rules, and __attribute__((noreturn)) is part of the type system, and [[noreturn]] isn't. This avoids breaking existing code, but is bound to confuse users when they try to change syntax (likely with macros such as those you mention) if they happened to be using cases where the two syntaxes bind differently. When you have two similar but incompatible ways of doing something, you have a no win situation for users - and it's not WG14's job to create work for front-end writers implementing the different modes for compatibility with different code, they should be helping users by avoiding needing the different modes. The fact that there are some problems with existing implementations (including bugs where it may not be at all clear what is actually the best thing to do), such as questions of exactly how attributes should interact with the type system, may mean that a standard version cannot be perfectly compatible with existing implementations in all cases. But by studying what implementations actually do, and what real code using these features does, and what problems have been noted in implementation over the years, it ought to be possible to come up with something friendlier to users (and less ambitious) than the existing proposals. So far however the proposers of attributes standardization do not seem to have been paying any attention to my points [...] Issues we've observed in practice with how attributes interact with the type system include those described in <http://gcc.gnu.org/ml/gcc/2006-10/msg00318.html> and <http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01056.html>. -- Joseph S. Myers jos...@codesourcery.com
