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<title>Issue 4018: ranges::to's copy branch is underconstrained</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#New">New</a> status.</em></p>
<h3 id="4018"><a href="lwg-active.html#4018">4018</a>. <code>ranges::to</code>'s copy branch is underconstrained</h3>
<p><b>Section:</b> 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-11-25 <b>Last modified:</b> 2024-03-11</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#range.utility.conv.to">active issues</a> in [range.utility.conv.to].</p>
<p><b>View all other</b> <a href="lwg-index.html#range.utility.conv.to">issues</a> in [range.utility.conv.to].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Unlike other branches that return a prvalue <code>C</code>, this branch's <code>C</code> is returned by elidable move, 
indicating that <code>C</code> needs to be move constructible (<a href="https://godbolt.org/z/dfqnx6Txq">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;

struct nonmovable {
  nonmovable() = default;
  nonmovable(const nonmovable&amp;) = delete;
  nonmovable&amp; operator=(const nonmovable&amp;) = delete;
};

template&lt;class T&gt;
struct nonmovable_vector : std::vector&lt;T&gt;, nonmovable { };

int main() {
  int arr[] = {42};
  auto v = std::ranges::to&lt;nonmovable_vector&lt;int&gt;&gt;(arr); // <span style="color:red;font-weight:bolder">hard error</span>
}
</pre></blockquote>

<p><i>[2024-03-11; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-4018"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4964" title=" Working Draft, Programming Languages — C++">N4964</a>.
</p>

<ol>

<li><p>Modify 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Mandates</i>: <code>C</code> is a cv-unqualified class type.
</p>
<p>
-2- <i>Returns</i>: An object of type <code>C</code> constructed from the elements of <code>r</code> in the following manner:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; If <code>C</code> does not satisfy <code>input_range</code> or 
<code>convertible_to&lt;range_reference_t&lt;R&gt;, range_value_t&lt;C&gt;&gt;</code> is <code>true</code>:</p>
<ol style="list-style-type: none">
<li><p>(2.1.1) &mdash; [&hellip;]</p></li>
<li><p>(2.1.2) &mdash; [&hellip;]</p></li>
<li><p>(2.1.3) &mdash; [&hellip;]</p></li>
<li><p>(2.1.4) &mdash; Otherwise, if</p>
<ol style="list-style-type: none">
<li><p><ins>(2.1.4.?) &mdash; <code>move_constructible&lt;C&gt;</code> is <code>true</code>,</ins></p></li>
<li><p>(2.1.4.1) &mdash; <code>constructible_from&lt;C, Args...&gt;</code> is <code>true</code>, and</p></li>
<li><p>(2.1.4.2) &mdash; <code><i>container-insertable</i>&lt;C, range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:</p>
<blockquote><pre>
C c(std::forward&lt;Args&gt;(args)...);
if constexpr (sized_range&lt;R> &amp;&amp; <i>reservable-container</i>&lt;C&gt;)
  c.reserve(static_cast&lt;range_size_t&lt;C&gt;&gt;(ranges::size(r)));
ranges::copy(r, <i>container-inserter</i>&lt;range_reference_t&lt;R&gt;&gt;(c));
</pre></blockquote></li>
</ol>
</li>
<li><p>(2.1.5) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol></li>
<li><p>(2.2) &mdash; [&hellip;]</p></li>
<li><p>(2.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>

</blockquote>
</blockquote>
</li>

</ol>





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