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<title>Issue 3913: ranges::const_iterator_t&lt;range R&gt; fails to accept arrays of unknown bound</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="3913"><a href="lwg-closed.html#3913">3913</a>. <code>ranges::const_iterator_t&lt;range R&gt;</code> fails to accept arrays of unknown bound</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> Arthur O'Dwyer <b>Opened:</b> 2023-03-28 <b>Last modified:</b> 2023-11-07</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#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>ranges::iterator_t&lt;T&gt;</code> is unconstrained; but in contrast,
<code>ranges::const_iterator_t&lt;T&gt;</code> constrains <code>T</code> to be a <code>range</code>,
i.e., it must have an <code>end</code>.
<pre><code>
    static_assert(std::is_same_v&lt;std::ranges::iterator_t&lt;int(&amp;)[2]&gt;, int*&gt;);
    static_assert(std::is_same_v&lt;std::ranges::iterator_t&lt;int(&amp;)[]&gt;, int*&gt;);  // OK
    static_assert(std::is_same_v&lt;std::ranges::const_iterator_t&lt;int(&amp;)[2]&gt;, const int*&gt;);
    static_assert(std::is_same_v&lt;std::ranges::const_iterator_t&lt;int(&amp;)[]&gt;, const int*&gt;);  // hard error
</code></pre>
</p>

<p><i>[2023-06-01; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll. Several votes for NAD.
Send to SG9 to be looked at with <a href="https://wg21.link/P2836" title=" std::basic_const_iterator should follow its underlying type's convertibility">P2836</a>.
</p>

<p><i>[Kona 2023-11-07; SG9]</i></p>

<p>
SG9 considered this and noted it's obsolete due to LWG <a href="lwg-defects.html#3946" title="The definition of const_iterator_t should be reworked (Status: WP)">3946</a><sup><a href="https://cplusplus.github.io/LWG/issue3946" title="Latest snapshot">(i)</a></sup>.
To support this case now would require changing <code>ranges::cbegin</code>
to add a special case for arrays of unknown bound, which would require a paper.
</p>

<p><i>[Kona 2023-11-07; Status changed: Open &rarr; NAD.]</i></p>




<p id="res-3913"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4944" title=" Working Draft, Standard for Programming Language C++">N4944</a>.
</p>

<ol>
<li>
<p>Modify 25.2 <a href="https://wg21.link/ranges.syn">[ranges.syn]</a> as indicated:</p>

<blockquote><pre><code>
  template&lt;class T&gt;
    using iterator_t = decltype(ranges::begin(declval&lt;T&amp;&amp;gt;()));    <i>// freestanding</i>
  template&lt;range R&gt;
    using sentinel_t = decltype(ranges::end(declval&lt;R&amp;&amp;gt;()));      <i>// freestanding</i>
  template&lt;<del>range</del><ins>class</ins> R&gt;
    using const_iterator_t = const_iterator&lt;iterator_t&lt;R&gt;&gt;;       <i>// freestanding</i>
  template&lt;range R&gt;
    using const_sentinel_t = const_sentinel&lt;sentinel_t&lt;R&gt;&gt;;       <i>// freestanding</i>
</code></pre></blockquote>
</li>
</ol>






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