Support #elifdef and #elifndef in c.vim

[jwakely]

C2x and C++23 add these new preprocessor conditionals, as shorter forms of #elif defined and #elif !defined.

Fixes #13667

[jwakely]

This is the minimal change to make vim recognize them, so that syntax highlighting works, and you can jump between them using %. I have no idea if I should have also changed cCppOutIf2, cCppInElse, etc. because I don't know what they're used for.

[jwakely]

Another way to define that regex would be:

start="^\s*\zs\%(%:\|#\)\s*\%(\(el\)\?\(if\|ifdef\|ifndef\)\)\>"

Since each of if, ifdef, and ifndef now has an "elif" form, we can just give them all an optional el prefix.

[dkearns]

We were waiting for C23 to be released before including those changes. As these directives are already supported by GCC I think they should be conditionally included if the "c_gnu" flag is set.

cCpp{In,Out}* syntax groups are used to highlight 'disabled' code as comments. So these changes should, ideally, be applied there as well. See :help c_no_if0

[jwakely]

I don't think waiting makes sense. Both C2x and C++23 have been approved and are just awaiting publication now. The content is final, and these new preprocessor tokens have 0% chance of being removed.

And it's not a GCC extension. GCC, Clang and Intel icx all support it in their latest production releases:
https://godbolt.org/z/Y4zoaE1Ed
Only MSVC doesn't yet:
https://en.cppreference.com/w/cpp/compiler_support#cpp23

[jwakely]

cCpp{In,Out}* syntax groups are used to highlight 'disabled' code as comments. So these changes should, ideally, be applied there as well. See :help c_no_if0

OK, I'll try to do that too ...

[dkearns]

I don't think waiting makes sense. Both C2x and C++23 have been approved and are just awaiting publication now. The content is final, and these new preprocessor tokens have 0% chance of being removed.

OK, if it's locked down I guess there's no reason not to add it now. There's a few other C23 related changes queued as PRs and similar that I'll try and round up soon.

I gather you noticed there's a series of version flags and we'll need a new one, c_no_c23?

[chrisbra]

please ping me, once this is ready.