On Thu, Nov 13, 2025 at 2:00 PM Jonathan Wakely <cxx_at_[hidden]> wrote:
>
>
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> On Thu, 13 Nov 2025 at 13:18, Aaron Ballman <aaron_at_[hidden]> wrote:
>>
>> Responding to two separate threads below.
>>
>> On Wed, Nov 12, 2025 at 3:02 PM Jonathan Wakely <cxx_at_[hidden]> wrote:
>> > As I understand it, the whole point of the current specs is to allow implementers to have freedom to provide helpful diagnostics if they choose to. You seem to be saying "don't let me decide what helps my users, force me to implement a particular behaviour".
>>
>> Not really, no.
>>
>> The intent from WG14 (AIUI) was that we should continue to accept code
>> that was correct in previous versions of C and start accepting only a
>> single argument of va_list type, and nothing else. I think the C
>> wording is close to capturing that intent, but it's not fully clear
>> whether Clang is acting in a non-conforming manner here (in C) by
>> rejecting this code which GCC accepts: https://godbolt.org/z/zq6jnqedM
>> What I'm gathering from this discussion is that both Clang and GCC's
>> behavior is conforming,
>>
>> I do not think the C++ wording matches the intent from C because it
>> explicitly discards all tokens. I'd like to see the two standards
>> agree on the behavior here, and I think the tokens should be expanded
>> and not discarded because discarding the tokens leads to a specified
>> behavior for code which I think should be ill-formed in C++;
>> basically, I think Clang's behavior on the example above should apply
>> in C++ but per the current wording, Clang is non-conforming. The
>> changes LWG made in https://cplusplus.github.io/LWG/issue4388 don't
>> quite accomplish what I was hoping for. The new words say "and any of
>> the second or subsequent arguments expands to include" but the old
>> words "The preprocessing tokens comprising the second and subsequent
>> arguments to va_start (if any) are discarded." still remain.
>
>
> That was very much intentional.
>
> The change is to say that certain misuses are ill-formed; no diagnostic required. There was no intention to also change it to not discard the tokens. We just don't want to imply that discarding the tokens means that it's somehow OK to use unmatched parentheses or completely invalid tokens there, and expect compilers to handle that somehow and make the program well-formed.

Which is good, I think the changes that LWG made are correct and
necessary, just insufficient for addressing a separate concern.

>> So I read
>> that as requiring us to expand __COUNTER__ which would increment it,
>> but it also requires us to not reject code from my linked example
>> because the resulting expanded tokens are discarded. I think it should
>> say the (non-preprocessing) tokens are not evaluated.
>
>
> I still find it extremely hard to care about anybody dumb enough to do *anything* in the second argument to va_start. We've already spent more time on these corner cases than any users will ever spend being confused or upset by this aspect of va_start.

I get that you don't care. I don't get why you insist I don't care.
This study group's bailiwick is to address needless incompatibilities
between the languages, that's what I'm asking for.

> If you warn about anything except an identifier as the second argument, is that good enough?

No.

va_start(list, this_identifier_does_not_exist) should not compile for
the same reason that 1 || this_identifier_does_not_exist does not
despite not evaluating the second operand in either case. The current
C++ wording prevents an implementation from rejecting that code
because of the normative requirement to *discard* the tokens for that
second argument to va_start.

Anyhow, if SG22 doesn't care about this incompatibility, that's fine.
Parts of this discussion were useful enough for me to figure out what
Clang will do.

Thanks!

~Aaron