CWG1790 Ellipsis following function parameter pack

[jfbastien]

Migrating from https://isocpp.org/files/papers/P1018R18.html#issues
CWG1790

Section: 9.3.4.6 [dcl.fct] Status: extension Submitter: Daryle Walker Date: 2013-10-01
Although the current wording permits an ellipsis to immediately follow a function parameter pack, it is not clear that the facilities permit access to the ellipsis arguments.

Rationale (June, 2014):

CWG felt that this is a question of language design and thus should be considered by EWG before any action.

Although the current wording permits an ellipsis to immediately follow a function parameter pack, it is not clear that the facilities permit access to the ellipsis arguments.

The problem here (which is not explained in the issue) is: how do you supply the name of the last parameter before the ellipsis to va_start? You can’t put the name of a pack there (it wouldn’t be expanded) and there’s no way to name the last element of the pack (nor to deal with the case where the pack is empty).

Meeting: (notes) 3 options: fix wording around “last parameter”, remove facility entirely (either va_start or function declarator), try to invent a language facility. JF emailed EWG to see if anyone has a strong preference, or if we should send back to CWG to fix wording, long discussion.

Michael Spertus: I am willing to commit to including and analysis on this in an upcoming paper on parameter packs.

JF followed up with Michael and Barry, no response.

[jensmaurer]

C23 removes the requirement that the last parameter be named for va_start.

All that remains is updating our specification of va_start to follow, which is handled by P2537 Relax va_start Requirements to Match C #1200.