Uh-oh. Using

#ifdef __VA_OPT__

as a feature test system is problematic because we previously said that __VA_OPT__ is ill-formed if it appears anywhere other than in a #define for a variadic macro. Several existing C++20 implementations reject that #ifdef.

Perhaps we don't need a feature-test macro, though:

#define HAS_VA_OPT_IMPL___VA_OPT__(...) 0
#define HAS_VA_OPT_IMPL(A, ...) HAS_VA_OPT_IMPL_ ## __VA_OPT__(YES)


... seems to work out OK.

On Tue, Jan 26, 2021 at 6:28 AM Ben Craig <ben.craig@ni.com> wrote:



From: Liaison <liaison-bounces@lists.isocpp.org> On Behalf Of Thomas Köppe via Liaison
Sent: Monday, January 25, 2021 5:21 PM
To: Richard Smith <richardsmith@google.com>
Cc: Thomas Köppe <tkoeppe@google.com>; liaison@lists.isocpp.org; sg10@lists.isocpp.org
Subject: [EXTERNAL] Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)


The proposed solution seems elegant to me. If you say that's implementable, I'd be in favour of that.


I have very regrettably not sent an updated paper to WG14 for this feature, it's one of the things on my list. I shall prioritize getting it done!


On Mon, 25 Jan 2021 at 23:02, Richard Smith <richardsmith@google.com> wrote:

[+WG14 liaison list]



On Mon, Jan 25, 2021 at 2:59 PM Richard Smith <richardsmith@google.com> wrote:



SD-FeatureTest doesn't mention P0306. I think a feature test macro would be useful here, to allow projects to incrementally adopt the new functionality. Example:


#if __cpp_va_opt

#define FOO(a, ...) f(a __VA_OPT__(,) __VA_ARGS__)

#elif __GNUC__

#define FOO(a, ...) f(a , ## __VA_ARGS__)


// Hopefully we get the MSVC implicit comma deletion behavior.

#define FOO(a, ...) f(a , __VA_ARGS__)



Note that the __GNUC__ extension is enabled by default, even in conforming modes, in GCC, Clang, and ICC. However, after the adoption of P0306, it's no longer a conforming extension, so presumably it will be phased out at some point, and uses of the feature-test macro, such as in the above example, are going to become necessary.


Regarding the name of the macro: this functionality is shared with C, and as such, a __cpp_* name is probably not ideal. However, there's another interesting option: we could use


#ifdef __VA_OPT__



as the feature test mechanism. This doesn't appear to conflict with anything else, and is in line with our feature test mechanism for __has_cpp_attribute and __has_include.


So that's my suggestion: #ifdef/#ifndef/defined should treat `__VA_OPT__` as if it were the name of a defined macro.