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<title>Issue 3435: three_way_comparable_with&lt;reverse_iterator&lt;int*&gt;, reverse_iterator&lt;const int*&gt;&gt;</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="3435"><a href="lwg-defects.html#3435">3435</a>. <code>three_way_comparable_with&lt;reverse_iterator&lt;int*&gt;, reverse_iterator&lt;const int*&gt;&gt;</code></h3>
<p><b>Section:</b> 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a>, 24.5.1.4 <a href="https://wg21.link/reverse.iter.cons">[reverse.iter.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-04-26 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#move.iter.cons">issues</a> in [move.iter.cons].</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>
Despite that <code>reverse_iterator&lt;int*&gt;</code> and <code>reverse_iterator&lt;const int*&gt;</code> are comparable with 
<code>&lt;=&gt;</code>, <code>three_way_comparable_with&lt;reverse_iterator&lt;int*&gt;, reverse_iterator&lt;const int*&gt;&gt;</code> 
is <code>false</code>. This unfortunate state of affairs results from the absence of constraints on <code>reverse_iterator</code>'s 
converting constructor: both <code>convertible_to&lt;reverse_iterator&lt;int*&gt;, reverse_iterator&lt;const int*&gt;&gt;</code>
and <code>convertible_to&lt;reverse_iterator&lt;const int*&gt;, reverse_iterator&lt;int*&gt;&gt;</code> evaluate to <code>true</code>, 
despite that <code>reverse_iterator&lt;int*&gt;</code>'s converting constructor template is ill-formed when instantiated 
for <code>reverse_iterator&lt;const int*&gt;</code>. This apparent bi-convertibility results in ambiguity when trying to 
determine <code>common_reference_t&lt;const reverse_iterator&lt;int*&gt;&amp;, const reverse_iterator&lt;const int*&gt;&amp;&gt;</code>, 
causing the <code>common_reference</code> requirement in <code>three_way_comparable_with</code> to fail.
<p/>
I think we should correct this by constraining <code>reverse_iterator</code>'s conversion constructor (and converting 
assignment operator, while we're here) correctly so we can use the concept to determine when it's ok to compare 
specializations of <code>reverse_iterator</code> with <code>&lt;=&gt;</code>.
<p/>
<code>move_iterator</code> has similar issues due to its similarly unconstrained conversions. We should fix both similarly.
</p>

<p><i>[2020-07-17; Priority set to 2 in telecon]</i></p>


<p><i>[2020-07-26; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector discussions.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3435"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 24.5.4.4 <a href="https://wg21.link/move.iter.cons">[move.iter.cons]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> this incorporates and supercedes the P/R of LWG <a href="lwg-defects.html#3265" title="move_iterator's conversions are more broken after P1207 (Status: C++23)">3265</a><sup><a href="https://cplusplus.github.io/LWG/issue3265" title="Latest snapshot">(i)</a></sup>.]
</p>
</blockquote>

<blockquote>
<pre>
constexpr move_iterator();
</pre>
<blockquote>
<p>
-1- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, v</del><ins>V</ins>alue-initializ<ins>es</ins><del>ing</del> 
<code>current</code>. <del>Iterator operations applied to the resulting iterator have defined behavior if and only if 
the corresponding operations are defined on a value-initialized iterator of type <code>Iterator</code>.</del>
</p>
</blockquote>
<pre>
constexpr explicit move_iterator(Iterator i);
</pre>
<blockquote>
<p>
-2- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>std::move(i)</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-3- <del><i>Mandates:</i> <code>U</code> is convertible to <code>Iterator</code></del><ins><i>Constraints:</i> 
<code>is_same_v&lt;U, Iterator&gt;</code> is <code>false</code> and <code>const U&amp;</code> models 
<code>convertible_to&lt;Iterator&gt;</code></ins>.
<p/>
-4- <i>Effects:</i> <del>Constructs a <code>move_iterator</code>, i</del><ins>I</ins>nitializ<ins>es</ins><del>ing</del> 
<code>current</code> with <code>u.<ins>current</ins><del>base()</del></code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr move_iterator&amp; operator=(const move_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
-5- <del><i>Mandates:</i> <code>U</code> is convertible to <code>Iterator</code></del><ins><i>Constraints:</i> 
<code>is_same_v&lt;U, Iterator&gt;</code> is <code>false</code>, <code>const U&amp;</code> models 
<code>convertible_to&lt;Iterator&gt;</code>, and <code>assignable_from&lt;Iterator&amp;, const U&amp;&gt;</code> 
is modeled</ins>.
<p/>
-6- <i>Effects:</i> Assigns <code>u.<ins>current</ins><del>base()</del></code> to <code>current</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.5.1.4 <a href="https://wg21.link/reverse.iter.cons">[reverse.iter.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class U&gt; constexpr reverse_iterator(const reverse_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
<ins>-?- <i>Constraints:</i> <code>is_same_v&lt;U, Iterator&gt;</code> is <code>false</code> and 
<code>const U&amp;</code> models <code>convertible_to&lt;Iterator&gt;</code>.</ins>
<p/>
-3- <i>Effects:</i> Initializes <code>current</code> with <code>u.current</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt;
  constexpr reverse_iterator&amp;
    operator=(const reverse_iterator&lt;U&gt;&amp; u);
</pre>
<blockquote>
<p>
<ins>-?- <i>Constraints:</i> <code>is_same_v&lt;U, Iterator&gt;</code> is <code>false</code>, 
<code>const U&amp;</code> models <code>convertible_to&lt;Iterator&gt;</code>, and 
<code>assignable_from&lt;Iterator&amp;, const U&amp;&gt;</code> is modeled.</ins>
<p/>
-4- <i>Effects:</i> Assigns <code>u.<ins>current</ins><del>base()</del></code> to <code>current</code>.
<p/>
-5- <i>Returns:</i> <code>*this</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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