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<title>Issue 4082: views::concat(r) is well-formed when r is an output_range</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#WP">WP</a> status.</em></p>
<h3 id="4082"><a href="lwg-defects.html#4082">4082</a>. <code>views::concat(r)</code> is well-formed when <code>r</code> is an <code>output_range</code></h3>
<p><b>Section:</b> 25.7.18.1 <a href="https://wg21.link/range.concat.overview">[range.concat.overview]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-04-27 <b>Last modified:</b> 2024-07-08</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, <code>views::concat</code> will return <code>views::all(r)</code> when it takes only one argument, 
which only requires that the type of <code>r</code> models <code>viewable_range</code> which includes <code>output_range</code>:
</p>
<blockquote><pre>
std::vector&lt;int&gt; v;
auto r = std::views::counted(std::back_inserter(v), 3);
auto c = std::views::concat(r); // <span style="color:red;font-weight:bolder">well-formed</span>
</pre></blockquote>
<p>
Since <code>concat_view</code> requires all ranges to be <code>input_range</code>, this seems inconsistent. 
We should reject the above just like <code>views::zip_transform</code> still requires <code>F</code> to be 
<code>move_constructible</code> in the case of an empty pack.
</p>

<p><i>[2024-05-08; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

<p><i>[St. Louis 2024-06-29; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4082"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4981" title=" Working Draft, Programming Languages — C++">N4981</a>.
</p>

<ol>
<li><p>Modify 25.7.18.1 <a href="https://wg21.link/range.concat.overview">[range.concat.overview]</a> as indicated:</p>

<blockquote>
<p>
-2- The name <code>views::concat</code> denotes a customization point object (16.3.3.3.5 <a href="https://wg21.link/customization.point.object">[customization.point.object]</a>).
Given a pack of subexpressions <code>Es...</code>, the expression <code>views::concat(Es...)</code> is expression-equivalent to
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; <code>views::all(Es...)</code> if <code>Es</code> is a pack with only one element <ins>whose type models 
<code>input_range</code></ins>,</p></li>
<li><p>(2.2) &mdash; otherwise, <code>concat_view(Es...)</code>.</p></li>
</ol>

</blockquote>
</li>

</ol>





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