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Subject: Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)
From: Barry Revzin (barry.revzin_at_[hidden])
Date: 2021-01-28 09:30:08


On Thu, Jan 28, 2021 at 4:23 AM Jonathan Wakely via SG10 <
sg10_at_[hidden]> wrote:

>
>
> On Thu, 28 Jan 2021 at 10:18, Jonathan Wakely <cxx_at_[hidden]> wrote:
>
>>
>>
>> On Thu, 28 Jan 2021 at 10:15, Ville Voutilainen <
>> ville.voutilainen_at_[hidden]> wrote:
>>
>>> On Thu, 28 Jan 2021 at 12:13, Jonathan Wakely via SG10
>>> <sg10_at_[hidden]> wrote:
>>> >
>>> >
>>> >
>>> > On Wed, 27 Jan 2021 at 20:54, Barry Revzin via Liaison <
>>> liaison_at_[hidden]> 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.
>>
>
> 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.
> #else
> // Maybe it's actually available, but we can't be sure.
> // Assume it isn't.
> #endif
>
> So the question is whether to spell it __STDC_VA_OPT for WG14 compat, or
> __cpp_va_opt.
>
> --
> SG10 mailing list
> SG10_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/sg10

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?).

Barry



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