Mailing list review 2021-11-29

• The proposed change breaks both CTAD for pairs of iterators (now deduced to a list), span, and makes overloads set with both a span and a const vector& ambiguous. This needs to be better explored in the paper
• The paper motivation needs to be made clearer. The intent of the authors is to be able to replace vector by span in the parameter list and not to have to change the caller code at all
• The paper should discuss constructing from a templated initializer constructor, which is fairly frequent in numerical code
• This is fairly dangerous to add after the fact. Is it something that we want as a DR? In any case this issue should be visited fairly soon.
• The author should explore whether a feature macro is necessary.

My feeling is that the mailing list reviewers are fairly split on the issue.

We should revisit this issue fairly quickly with a bigger group (telecon) , either before the February feature freeze or shortly thereafter.
In the meantime, we encourage the author to provide an analysis of the potential breakage of existing code

P2447R1 std::span and the missing constructor (Federico Kircheis)

P2447R2 std::span and the missing constructor (Federico Kircheis)

2022-07-26 Library Evolution Telecon

P2447R2: std::span and the missing constructor

2022-07-26 Library Evolution Telecon Minutes

Chair: Nevin Liber

Minute Taker: Ben Craig

Champion: Federico Kircheis

Summary

There were concerns over whether or not adding the constructor was worth the cost (ease of creating a dangling reference, constructor ambiguity, etc.). Some suggested that if it were more explicit, either wrapping it with something like as_span(init_list) or using a literal (which would require a core language change), it might be more palatable, but that goes against other design decisions of span.

POLL: We should promise more committee time to pursuing P2447R2, knowing that our time is scarce and this will leave less time for other work

Attendance: 19

# of Authors: 1

Author Position: WF

Outcome: Weakly in favor

POLL: Send P2447R2 (std::span and the missing constructor) to LWG for C++26 classified as B2 (to be confirmed with a Library Evolution electronic poll)

Attendance: 19

# of Authors: 1

Author Position: SF

Outcome: No consensus

No consensus on the current design, and weakly in favor of exploring explicit designs.

Next Steps

Need an update to this paper exploring other designs if the author should so choose to pursue it.

P2447R3 std::span over an initializer list (Arthur O'Dwyer, Federico Kircheis)

P2447R4 will be in the 2023-05 mailing.

2023-09-12 Library Evolution Telecon

LEWG discussed "P2447R4: Freestanding Library: inout expected span" on telecon (Full Minutes)

Champion: Arthur O'Dwyer

Chair: Fabio Fracassi

Minute Taker: Ben Craig

Summary

POLL: Forward P2447R4(span over braced initalizer_list) to LWG for C++26(and as a defect) (to be confirmed by electronic polling)

Attendance: 18

# of Authors: 1

Author Position: SF

Outcome: No consensus

POLL: Forward P2447R4(span over braced initalizer_list) to LWG for C++26 (to be confirmed by electronic polling)

Attendance: 18

# of Authors: 1

Author Position: SF

Outcome: Weak consensus

WA: I think this kind of small improvement doesn't belong to the standard, as it only helps function invocations while introducing breaking changes

Some concerns exist as to whether this belongs to the library, or can be achieved as an implementation improvement. The paper was forwarded to LWG for C++26, but there was no consensus to forward it as a DR for C++23.

As this is a breaking change, implementers will be informed.

Next Steps

The paper was forwarded to LWG for C++26 (to be confirmed by an electronic poll).

P2447R4 was added to LEWG 2023 September electronic poll paper (P2972R0)

P2447R5 std::span over an initializer list (Arthur O'Dwyer, Federico Kircheis)

P2447R4 was forwarded to LWG on LEWG 2023 September electronic poll (P3020R0)

Reviewed and approved Kona 2023

https://wiki.edg.com/bin/view/Wg21kona2023/LibraryWorkingGroup

Poll: put P2447r6 into C++26?

|F|A|N|
|6|2|2|

LWG 2023-11-10: Approved for plenary straw polls in Kona.

P2447R6 std::span over an initializer list (Arthur O'Dwyer, Federico Kircheis)