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<title>Issue 3860: range_common_reference_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="3860"><a href="lwg-defects.html#3860">3860</a>. <code>range_common_reference_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> 2023-01-24 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</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>
For the alias template <code>iter_<i>meow</i>_t</code> in <code>&lt;iterator&gt;</code>, there are almost all corresponding 
<code>range_<i>meow</i>_t</code> in <code>&lt;ranges&gt;</code>, except for <code>iter_common_reference_t</code>, which is used to 
calculate the common reference type shared by <code>reference</code> and <code>value_type</code> of the iterator.
<p/>
Given that it has a highly similar formula form to <code>iter_const_reference_t</code>, and the latter has a corresponding sibling, 
I think we should add a <code>range_common_reference_t</code> for <code>&lt;ranges&gt;</code>.
<p/>
This increases the consistency of the two libraries and simplifies the text of getting common reference from a range. 
Since C++23 brings proxy iterators and <code>tuple</code> enhancements, I believe such introduction can bring some value.
</p>

<p><i>[2023-02-06; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after seven votes in favour during reflector poll.
</p>

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



<p id="res-3860"><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 25.2 <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;range R&gt;
    using range_reference_t = iter_reference_t&lt;iterator_t&lt;R&gt;&gt;;                      // <i>freestanding</i>
  template&lt;range R&gt;
    using range_const_reference_t = iter_const_reference_t&lt;iterator_t&lt;R&gt;&gt;;          // <i>freestanding</i>
  template&lt;range R&gt;
    using range_rvalue_reference_t = iter_rvalue_reference_t&lt;iterator_t&lt;R&gt;&gt;;        // <i>freestanding</i>
  <ins>template&lt;range R&gt;
    using range_common_reference_t = iter_common_reference_t&lt;iterator_t&lt;R&gt;&gt;;        // <i>freestanding</i></ins>
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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