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<title>Issue 3847: ranges::to can still return views</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#C++23">C++23</a> status.</em></p>
<h3 id="3847"><a href="lwg-defects.html#3847">3847</a>. <code>ranges::to</code> can still return views</h3>
<p><b>Section:</b> 25.5.7.2 <a href="https://wg21.link/range.utility.conv.to">[range.utility.conv.to]</a>, 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-01-06 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>2
</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#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The intention of <code>ranges::to</code> is to construct a non-view object,
which is reflected in its constraint that object type <code>C</code> should not
model a <code>view</code>.
</p>
<p>
The specification allows <code>C</code> to be a <code>const</code>-qualified type
which does not satisfy <code>view</code> such as <code>const string_view</code>,
making <code>ranges::to</code> return a dangling view. We should ban such cases.
</p>

<p><i>[2023-02-01; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
Just require C to be a cv-unqualified object type.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<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 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a> as indicated:</p>

<blockquote>
<pre>#include &lt;compare&gt;              // <i>see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>
#include &lt;initializer_list&gt;     // <i>see 17.11.2 <a href="https://wg21.link/initializer.list.syn">[initializer.list.syn]</a></i>
#include &lt;iterator&gt;             // <i>see 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a></i>

namespace std::ranges {
  […]
  // <i>25.5.7 <a href="https://wg21.link/range.utility.conv">[range.utility.conv]</a>, range conversions</i>
  template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;<ins>remove_cv_t&lt;</ins>C<ins>&gt;</ins>&gt;)
    constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);                                          // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, input_range R, class... Args&gt;
    constexpr auto to(R&amp;&amp; r, Args&amp;&amp;... args);                                       // <i>freestanding</i>
  template&lt;class C, class... Args&gt; requires (!view&lt;<ins>remove_cv_t&lt;</ins>C<ins>&gt;</ins>&gt;)
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  template&lt;template&lt;class...&gt; class C, class... Args&gt;
    constexpr auto to(Args&amp;&amp;... args);                                              // <i>freestanding</i>
  […]
}
</pre>
</blockquote>
</li><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;<ins>remove_cv_t&lt;</ins>C<ins>&gt;</ins>&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>
[…]
</blockquote>
</blockquote>
</li>
<li><p>Modify 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> as indicated:</p>
<blockquote>
<pre>template&lt;class C, class... Args&gt; requires (!view&lt;<ins>remove_cv_t&lt;</ins>C<ins>&gt;</ins>&gt;)
  constexpr auto to(Args&amp;&amp;... args);
template&lt;template&lt;class...&gt; class C, class... Args&gt;
  constexpr auto to(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Returns</i>: A range adaptor closure object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>) <code>f</code> that is a perfect forwarding call wrapper (22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>) with the following properties:
</p>
[…]
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2023-02-02; Jonathan provides improved wording]</i></p>


<p><i>[Issaquah 2023-02-08; LWG]</i></p>

<p>
Unanimous consent to move to Immediate.
This also resolves LWG <a href="lwg-defects.html#3787" title="ranges::to's template parameter C should not be a reference type (Status: Resolved)">3787</a><sup><a href="https://cplusplus.github.io/LWG/issue3787" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3847"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</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><ins>-?- <i>Mandates</i>: <code>C</code> is a cv-unqualified class type.</ins></p>
<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>
[…]
</blockquote>
</blockquote>
</li>
<li><p>Modify 25.5.7.3 <a href="https://wg21.link/range.utility.conv.adaptors">[range.utility.conv.adaptors]</a> as indicated:</p>
<blockquote>
<pre>template&lt;class C, class... Args&gt; requires (!view&lt;C&gt;)
  constexpr auto to(Args&amp;&amp;... args);
template&lt;template&lt;class...&gt; class C, class... Args&gt;
  constexpr auto to(Args&amp;&amp;... args);
</pre>
<blockquote>
<p><ins>-?- <i>Mandates</i>: For the first overload, <code>C</code> is a cv-unqualified class type.</ins></p>
<p>
-1- <i>Returns</i>: A range adaptor closure object (25.7.2 <a href="https://wg21.link/range.adaptor.object">[range.adaptor.object]</a>) <code>f</code> that is a perfect forwarding call wrapper (22.10.4 <a href="https://wg21.link/func.require">[func.require]</a>) with the following properties:
</p>
[…]
</blockquote>
</blockquote>
</li>
</ol>





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