DE288 24.07 Overspecification of return types of view adaptors P1739 P1391 P1394 P1664

[wg21bot]

By fully specifying the types returned by view adaptors, the standard
forces the return type of multiple chained view operations to be an
increasingly nested template. In general this is not avoidable, but for
certain combinations of input ranges and view adaptors, one can simply
create a modified object of the original type (e.g. with different
bounds). Not addressing this is a design flaw that needlessly
complicates working with views.

Proposed change:

1. Adopt P1739 and in this context also P1391 and P1394 which are
   strongly suggested by P1739. All three papers have been seen and
   approved by LEWG. P1739 cannot be adopted after C++20 without breaking API.
2. Discuss whether P1664 (or parts of it) should also be adopted. P1664
   generalises the notion of "reconstructible ranges" (those affected by
   P1739).
3. Evaluate whether any other combinations of range and view adaptor
   should get special treatment; any such changes after C++20 are breaking.generalises the notion of "reconstructible ranges" (those affected by
   P1739).
4. Evaluate whether any other combinations of range and view adaptor
   should get special treatment; any such changes after C++20 are breaking.

[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

[jensmaurer]

See also US272 #268.

[tituswinters]

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 -- related to US272 - same 4 papers.

[mclow]

LWG Friday morning Belfast. Accept. Note that P1739 is superseded by P1664

[mclow]

P1391 was adopted in Belfast.
P1394 was adopted in Belfast.
P1739 was superseded by P1664
P1664 was withdrawn in Belfast.
P1664R2 was in the post-Belfast mailing targeting C++23.