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<title>Issue 3768: possibly-const-range is overconstrained</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#NAD">NAD</a> status.</em></p>
<h3 id="3768"><a href="lwg-closed.html#3768">3768</a>. <code><i>possibly-const-range</i></code> is overconstrained</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#NAD">NAD</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2022-09-08 <b>Last modified:</b> 2022-11-30</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#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Due to the <code><i>possibly-const-range</i></code> constraint that the template parameter <code>R</code> 
must model <code>input_range</code>, this makes the <code>ranges::cend</code> that tries using it 
for meaningful <code>const</code> casting never be applied to an <code>output_range</code>, even though 
<code>const_sentinel</code> does not require the template parameter <code>S</code> to model 
<code>input_iterator</code>.
<p/>
This is unnecessary, we should relax its constraint.
</p>

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

<p>Tentatively NAD. "const output ranges don’t seem to be useful."</p>

<p><i>[2022-11-30 LWG telecon. Status changed: Tentatively NAD &rarr; NAD.]</i></p>



<p id="res-3768"><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> 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;]

  // <i>25.7.22 <a href="https://wg21.link/range.as.const">[range.as.const]</a>, as const view</i>
  template&lt;<del>input_</del>range R&gt;
    constexpr auto&amp; <i>possibly-const-range</i>(R&amp; r) {          // <i>exposition only</i>
      if constexpr (constant_range&lt;const R&gt; &amp;&amp; !constant_range&lt;R&gt;) {
        return const_cast&lt;const R&amp;&gt;(r);
      } else {
        return r;
      }
    } 
  
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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