On Thu, Jan 28, 2021 at 4:23 AM Jonathan Wakely via SG10 <sg10@lists.isocpp.org> wrote:

On Thu, 28 Jan 2021 at 10:18, Jonathan Wakely <cxx@kayari.org> wrote:

On Thu, 28 Jan 2021 at 10:15, Ville Voutilainen <ville.voutilainen@gmail.com> wrote:
On Thu, 28 Jan 2021 at 12:13, Jonathan Wakely via SG10
<sg10@lists.isocpp.org> wrote:
> On Wed, 27 Jan 2021 at 20:54, Barry Revzin via Liaison <liaison@lists.isocpp.org> wrote:
>> Eric Niebler actually asked this on StackOverflow a few years ago: https://stackoverflow.com/q/48045470/2069064
>> The accepted answer there is:
>> #define PP_THIRD_ARG(a,b,c,...) c
>> #define VA_OPT_SUPPORTED_I(...) PP_THIRD_ARG(__VA_OPT__(,),true,false,)
> Who is going to remember that without having to look it up though?

Is it going to be written so often that that becomes a major problem?

> The #ifdef __VA_OPT__ solution was my first thought, it's unfortunate we forbid it. If we can't have that then I think we do need a feature test macro. The voodoo above will make most developers wish they were using Rust.

If they're using VA_OPT, the cause is already lost.

And even if we add a feature test macro now (or allow #ifdef __VA_OPT__) there are still compilers that will reject it with an error (e.g. with -pedantic-errors in pre-C++20 modes). So maybe the ship has sailed and support this feature is already "untestable". You just have to know if your code can use it or not.

Actually that would be true for the #ifdef __VA_OPT__ solution (we could say it's allowed, but if you try to use it on today's shipping compilers, it's ill-formed) but if we add a new macro you can be conservative:

#ifdef __cpp_va_opt
// Use it.
// Maybe it's actually available, but we can't be sure.
// Assume it isn't.

So the question is whether to spell it __STDC_VA_OPT for WG14 compat, or __cpp_va_opt.

SG10 mailing list

So from the compilers I have available at my disposal (i.e. on compiler explorer):

* gcc trunk, clang trunk, and icc 21.1.9 have __VA_OPT__ in C++20 modes
* gcc even has it in C++17
* msvc 19.28 doesn't have it yet (that's the latest version up there)

If you try to use #ifdef __VA_OPT__:
* gcc gives you a warning, but with no label, so you can't disable it. If you compile with -Werror, you're done.
* clang gives you a warning that you can disable
* icc gives an error
* msvc is fine (since it doesn't support it yet, so no special rejection)

So yeah, #ifdef __VA_OPT__ probably not going to work out as a thing. But for __cpp_va_opt, is MSVC imminently going to add support for it? It may be not very informative (true means yes, false means try something else?).