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<title>Issue 3770: const_sentinel_t is missing</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="3770"><a href="lwg-defects.html#3770">3770</a>. <code>const_sentinel_t</code> is missing</h3>
<p><b>Section:</b> 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-05 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#ranges.syn">active issues</a> in [ranges.syn].</p>
<p><b>View all other</b> <a href="lwg-index.html#ranges.syn">issues</a> in [ranges.syn].</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 type alias pair <code>const_iterator</code> and <code>const_sentinel</code> 
and the factory function pair <code>make_const_iterator</code> and 
<code>make_const_sentinel</code> are defined in <code>&lt;iterator&gt;</code>, 
however, only <code>const_iterator_t</code> exists in <code>&lt;ranges&gt;</code>, 
which lacks <code>const_sentinel_t</code>, we should add it to ensure consistency.
<p/>
The proposed resolution also lifts the type constraint of <code>const_iterator_t</code> 
to <code>input_range</code> to make it consistent with <code>const_iterator</code>, 
which already requires that its template parameter must model 
<code>input_iterator</code>.
</p>

<p><i>[2022-09-23; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
<p/>
Comment from a reviewer:
</p>
<blockquote><p>
"I don't see why we should redundantly constrain <code>const_iterator_t</code>
with <code>input_range</code>; it's not as if we can make it more ill-formed
or more SFINAE-friendly than it is now."
</p></blockquote>

<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Previous resolution [SUPERSEDED]: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>, header <code>&lt;ranges&gt;</code> synopsis, 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 {
  [&hellip;]
  template&lt;class T&gt;
    using iterator_t = decltype(ranges::begin(declval&lt;T&amp;&gt;()));                      // freestanding
  template&lt;range R&gt;
    using sentinel_t = decltype(ranges::end(declval&lt;R&amp;&gt;()));                        // freestanding
  template&lt;<ins>input_</ins>range R&gt;
    using const_iterator_t = const_iterator&lt;iterator_t&lt;R&gt;&gt;;                         // freestanding
  <ins>template&lt;range R&gt;
    using const_sentinel_t = const_sentinel&lt;sentinel_t&lt;R&gt;&gt;;                         // freestanding</ins>
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2022-09-25; Daniel and Hewill provide alternative wording]</i></p>

<p>
The alternative solution solely adds the suggested <code>const_sentinel_t</code> alias template
and doesn't touch the <code>const_iterator_t</code> constraints.
</p>

<p><i>[2022-10-12; Reflector poll]</i></p>

<p>Set status to "Tentatively Ready" after seven votes in favour.</p>

<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3770"><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 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a>, header <code>&lt;ranges&gt;</code> synopsis, 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 {
  [&hellip;]
  template&lt;class T&gt;
    using iterator_t = decltype(ranges::begin(declval&lt;T&amp;&gt;()));                      // freestanding
  template&lt;range R&gt;
    using sentinel_t = decltype(ranges::end(declval&lt;R&amp;&gt;()));                        // freestanding
  template&lt;range R&gt;
    using const_iterator_t = const_iterator&lt;iterator_t&lt;R&gt;&gt;;                         // freestanding
  <ins>template&lt;range R&gt;
    using const_sentinel_t = const_sentinel&lt;sentinel_t&lt;R&gt;&gt;;                         // freestanding</ins>
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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