US272 24 API improvements for ranges P1664 P1739 P1391 P1394

[wg21bot]

24.4.2 Ranges [range.range];

§24.6.1.2 Class template empty_view [range.empty.view];

§24.6.3 Class template iota_view[range.iota.view];
§24.7.6.4 views::take [range.take.adaptor];
§24.7.8.3 views::drop [range.drop.adaptor]

Due to the ranges API being more or less fixed after shipment, API-breaking fixes have to be scheduled now rather than shipped in C++23. The papers p1664 and p1739 represent important and ultimately source-breaking changes to the ranges API. Unless shipped, these API optimizations will result in source code breaking at a later date if attempted to be fixed later, and also makes it impossible to reliably simplify the return value of ranges for a wide variety of current and future adaptors and algorithms. These changes are imperative for developing better APIs and it would be unfortunate to not be able to do them post-C++20 due to source breaking changes. All of the changes except for p1664's two new exposition-only concepts have been approved by LEWG during review of p1739. However, p1664 was discussed as part of p1739's approval and this comment was to be expected.

Proposed change:
The fix and its motivations have been formalized in two papers, [p1664 - Reconstructible Ranges] and [p1739 - Type preservation for forwarding Ranges for "subrange-y" views]. Further fixes are applied through Corentin Jabot's and Casey Carter's [p1391] and [p1394].

[jensmaurer]

• P1664 reconstructible_range - a concept for putting ranges back together P1664 reconstructible_range - a concept for putting ranges back together papers#578
• P1739 Type erasure for forwarding ranges in combination with "subrange-y" view adaptors P1739 Type erasure for forwarding ranges in combination with "subrange-y" view adaptors papers#503

[tituswinters]

LEWG in Belfast: LEWG in Belfast: We would still like all of these papers (P1391, P1394, P1739, and the new P1664) for C++20.
(Prioritization on these vs. other paper-sized NB comments will be shared with LWG chair when LEWG finishes NB comment processing.)

[JeffGarland]

LWG small group Belfast Wednesday - will need large group to process 4 papers.

[h-2]

P1391 and P1394 are independent (small) papers that are already on LWG's list.

P1739 is entirely contained/superseded by P1664.

edit: An updated draft of P1664 with snake_case concept syntax and minor fixes approved by LEWG will be posted here or to the LWG wiki.

[JeffGarland]

LWG approved P1664 in Belfast on Thu afternoon.

[mclow]

P1391 was adopted in Belfast.
P1394 was adopted in Belfast.
P1664 was withdrawn in Belfast.
P1664R2 was in the post-Belfast mailing targeting C++23.