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4082. views::concat(r) is well-formed when r is an output_range

Section: 26.7.18.1 [range.concat.overview] Status: WP Submitter: Hewill Kang Opened: 2024-04-27 Last modified: 2024-07-08

Priority: Not Prioritized

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Discussion:

Currently, views::concat will return views::all(r) when it takes only one argument, which only requires that the type of r models viewable_range which includes output_range:

std::vector<int> v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto c = std::views::concat(r); // well-formed

Since concat_view requires all ranges to be input_range, this seems inconsistent. We should reject the above just like views::zip_transform still requires F to be move_constructible in the case of an empty pack.

[2024-05-08; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[St. Louis 2024-06-29; Status changed: Voting → WP.]

Proposed resolution:

This wording is relative to N4981.

1. Modify 26.7.18.1 [range.concat.overview] as indicated:

   -2- The name views::concat denotes a customization point object (16.3.3.3.5 [customization.point.object]). Given a pack of subexpressions Es..., the expression views::concat(Es...) is expression-equivalent to

   1. (2.1) — views::all(Es...) if Es is a pack with only one element whose type models input_range,

   2. (2.2) — otherwise, concat_view(Es...).