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:
>
>
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> 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,)
>> #define VA_OPT_SUPPORTED VA_OPT_SUPPORTED_I(?)
>
>
> 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.