Specify when all with no senders

[ericniebler]

Issue by bustercopley
Friday Oct 14, 2022 at 18:19 GMT
Originally opened as brycelelbach/wg21_p2300_execution#3

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See NVIDIA/stdexec#482

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bustercopley included the following code: https://github.com/brycelelbach/wg21_p2300_execution/pull/3/commits

[inbal2l]

@ericniebler do I understand correctly that the 0 case is not supported? (I'm deducing from the PR, as the title is a bit unclear to me). If so, I think this is fixed in the wording of R10:
https://cplusplus.github.io/sender-receiver/execution.html#spec-execution.senders.adaptor.when_all
(actually, I think the behaviour for 0 is not specified in the design, but we're passed that and in the wording stage anyway).

[bustercopley]

@ericniebler do I understand correctly that the 0 case is not supported?

It is supported by the code in the NVIDIA/stdexec repository (since this commit), but disallowed by the wording of R10.

(I'm deducing from the PR, as the title is a bit unclear to me). If so, I think this is fixed in the wording of R10: https://cplusplus.github.io/sender-receiver/execution.html#spec-execution.senders.adaptor.when_all

The wording that makes the case of zero senders a special case is still there: in 34.9.11.11, ¶2, "The expressions when_all(sndrs...) and when_all_with_variant(sndrs...) are ill-formed if [...] sizeof...(sndrs) is 0". In my opinion it should be removed.

(actually, I think the behaviour for 0 is not specified in the design, but we're passed that and in the wording stage anyway).

The way I see it, making "sizeof...(sndrs) is 0" into a special case was not in the design in the first place. If it is not a special case then there is no need to mention the fact that it is not a special case. If I'm right about that, then no design fix is needed.

I would classify this as wording, bug (not design, needs-paper), and the wording changes that I suggest are linked in Eric's description (brycelelbach/wg21_p2300_execution#3). But I don't understand the procedures here, so I will leave it to the experts.