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<title>Issue 3845: ranges::to's from_range_t tag branch has the wrong constraint</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="3845"><a href="lwg-active.html#3845">3845</a>. <code>ranges::to</code>'s <code>from_range_t</code> tag branch has the wrong constraint</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> 2023-01-06 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>4
</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>
In bullet (1.1.2), <code>ranges::to</code> checks whether the container type
<code>C</code> models <code>constructible_from&lt;from_range_t, ...&gt;</code>
and constructs it via <code>C(from_range, ...)</code>.
</p>
<p>
Since <code>from_range</code> is a <code>constexpr</code> variable here,
it would be more accurate to constrain <code>C</code> to be constructible from
a const lvalue tag rather than an rvalue tag.
</p>

<p><i>[2023-02-02; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll.
Several votes for "Tentatively Ready", but also two objections, preferring NAD.
The proposed change would appear to bless unconventional uses of
<code>from_range</code>, and we don't want to support or encourage that.
The current wording is simpler, and works for the intended cases.
</p>



<p id="res-3845"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</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>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>(1.1) — If <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>(1.1.1) — If <code>constructible_from&lt;C, R, Args...&gt;</code> is <code>true</code>:</p>
    <blockquote><code>C(std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
    <li><p>(1.1.2) — Otherwise, if <code>constructible_from&lt;C, <ins>const </ins>from_range_t<ins>&amp;</ins>, R, Args...&gt;</code> is <code>true</code>:</p>
    <blockquote><code>C(from_range, std::forward&lt;R&gt;(r), std::forward&lt;Args&gt;(args)...)</code></blockquote></li>
    <li>[…]</li>
    </ol></li>
  </ol>
</blockquote>
</blockquote>
</li>
</ol>
  





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