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<title>Issue 3470: convertible-to-non-slicing seems to reject valid case</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="3470"><a href="lwg-defects.html#3470">3470</a>. <code><i>convertible-to-non-slicing</i></code> seems to reject valid case</h3>
<p><b>Section:</b> 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2020-07-26 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#range.subrange">issues</a> in [range.subrange].</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>
Consider
</p>
<blockquote><pre>
#include &lt;ranges&gt;

int main()
{
  int a[3] = { 1, 2, 3 };
  int* b[3] = { &amp;a[2], &amp;a[0], &amp;a[1] };
  auto c = std::ranges::subrange&lt;const int*const*&gt;(b);
}
</pre></blockquote>
<p>
The construction of <code>c</code> is ill-formed because <code><i>convertible-to-non-slicing</i>&lt;int**, const int*const*&gt;</code>
is <code>false</code>, although the conversion does not involve object slicing.
<p/>
I think <code>subrange</code> should allow such qualification conversion, just like <code>unique_ptr&lt;T[]&gt;</code> already does.
<p/>
(Given that this constraint is useful in more than one context, maybe it deserves a named type trait?)
</p>

<p><i>[2020-08-21; Reflector prioritization]</i></p>

<p>
Set priority to 3 after reflector discussions.
</p>
<p><i>[2021-05-19 Tim adds wording]</i></p>

<p>
The wording below, which has been implemented and tested on top of libstdc++,
uses the same technique we use for <code>unique_ptr</code>, <code>shared_ptr</code>,
and <code>span</code>. It seems especially appropriate to have feature parity between
<code>subrange</code> and <code>span</code> in this respect.
</p>

<p><i>[2021-06-23; Reflector poll]</i></p>

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

<p><i>[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3470"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.</p>

<ol>
<li><p>Modify 25.5.4 <a href="https://wg21.link/range.subrange">[range.subrange]</a> as indicated:</p>

<blockquote>
<pre>
namespace std::ranges {
  <ins>template&lt;class From, class To&gt;
    concept <i>uses-nonqualification-pointer-conversion</i> = <i>// exposition only</i>
      is_pointer_v&lt;From&gt; &amp;&amp; is_pointer_v&lt;To&gt; &amp;&amp;
      !convertible_to&lt;remove_pointer_t&lt;From&gt;(*)[], remove_pointer_t&lt;To&gt;(*)[]&gt;;</ins>

  template&lt;class From, class To&gt;
    concept <i>convertible-to-non-slicing</i> = <i>// exposition only</i>
      convertible_to&lt;From, To&gt; &amp;&amp;
      <ins>!<i>uses-nonqualification-pointer-conversion</i>&lt;decay_t&lt;From&gt;, decay_t&lt;To&gt;&gt;;</ins>
      <del>!(is_pointer_v&lt;decay_t&lt;From&gt;&gt; &amp;&amp;
      is_pointer_v&lt;decay_t&lt;To&gt;&gt; &amp;&amp;
      <i>not-same-as</i>&lt;remove_pointer_t&lt;decay_t&lt;From&gt;&gt;, remove_pointer_t&lt;decay_t&lt;To&gt;&gt;&gt;
      );</del>
  [&hellip;]
}
[&hellip;]
</pre>
</blockquote>
</li>
</ol>





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