[Bug c++/60512] would be useful if gcc implemented __has_feature similarly to clang

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60512

--- Comment #19 from Iain Sandoe <iains at gcc dot gnu.org> ---
(In reply to Alex Coplan from comment #18)
> (In reply to Iain Sandoe from comment #16)
> > 
> > AFAIR the main blocker to progress was trying to decide how to represent the
> > target/language/language version dependencies of the features and extensions
> > (there's a rudimentary scheme at the moment but it probably is not flexible
> > enough)
> 
> Can you elaborate on this a little? In particular I'd be interested to hear
> about cases where the target matters.

the first commit in my WIP branch is "generic support" (i.e. features that are
[intended to be] common to all targets)

the second commit identifies a set of features that (I would want to be)
supported on Darwin, but maybe not relevant to other targets.

Of course, we hope the first set to be the largest.

Targets like Darwin and Windows are more likely to need such things - where
compatibility with external ABI and system tools means we're trying to comply
with design decisions out of our control.

However, it is presumably conceivable that (say) variable length vector
features could be relevant to aarch64 and risc-v but not elsewhere (I'm
speculating here, so no concrete example or evidence).