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<title>Issue 3984: ranges::to's recursion branch may be ill-formed</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="3984"><a href="lwg-defects.html#3984">3984</a>. <code>ranges::to</code>'s recursion branch may be ill-formed</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#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-08-23 <b>Last modified:</b> 2024-04-02</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#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
When <code>r</code> is a nested range, <code>ranges::to</code> constructs the object recursively through <code>r | views::transform(...)</code>.
<p/>
However, the expression is ill-formed when the type of lvalue <code>r</code> does not model <code>viewable_range</code> 
(<a href="https://godbolt.org/z/98rxGz73Y">demo</a>):
</p>
<blockquote><pre>
#include &lt;ranges&gt;
#include &lt;vector&gt;
#include &lt;list&gt;

int main() {
  std::vector&lt;std::vector&lt;int&gt;&gt; v;
  auto r = std::views::all(std::move(v));
  auto l = std::ranges::to&lt;std::list&lt;std::list&lt;int&gt;&gt;&gt;(r); // <span style="color:red;font-weight:bolder">hard error in MSVC-STL and libc++</span>
}
</pre></blockquote>

<p><i>[2023-11-03; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
"Should be <code>std::forward&lt;R&gt;(r)</code> instead?"
</p>

<p><i>[Kona 2023-11-07; move to Ready]</i></p>


<p><i>[Tokyo 2024-03-23; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3984"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4958" title=" Working Draft, Programming Languages — C++">N4958</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>[&hellip;]</p></li>
</ol></li>
<li><p>(2.2) &mdash; Otherwise, if <code>input_range&lt;range_reference_t&lt;R&gt;&gt;</code> is <code>true</code>:
</p>
<blockquote>
<pre>
to&lt;C&gt;(<ins>ref_view(</ins>r<ins>)</ins> | views::transform([](auto&amp;&amp; elem) {
  return to&lt;range_value_t&lt;C&gt;&gt;(std::forward&lt;decltype(elem)&gt;(elem));
}), std::forward&lt;Args&gt;(args)...);
</pre>
</blockquote>
</li>
<li><p>(2.3) &mdash; Otherwise, the program is ill-formed.</p></li>
</ol>
</blockquote>
</blockquote>

</li>

</ol>





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