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<title>Issue 3953: iter_move for common_iterator and counted_iterator should return decltype(auto)</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#WP">WP</a> status.</em></p>
<h3 id="3953"><a href="lwg-defects.html#3953">3953</a>. <code>iter_move</code> for <code>common_iterator</code> and <code>counted_iterator</code> should return <code>decltype(auto)</code></h3>
<p><b>Section:</b> 24.5.5.7 <a href="https://wg21.link/common.iter.cust">[common.iter.cust]</a>, 24.5.7.7 <a href="https://wg21.link/counted.iter.cust">[counted.iter.cust]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-06-30 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Although the customized <code>iter_move</code> of both requires the underlying iterator <code>I</code> to be <code>input_iterator</code>, 
they still explicitly specify the return type as <code>iter_rvalue_reference_t&lt;I&gt;</code>,
which makes it always instantiated.
</p>
<p>
From the point of view that its validity is only specified in the <code>input_iterator</code> concept, it would be better to remove such 
unnecessary type instantiation, which does not make much sense for an <code>output_iterator</code> even if it is still valid.
</p>

<p><i>[2023-10-27; Reflector poll]</i></p>

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

<p><i>[2023-11-11 Approved at November 2023 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3953"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4950" title=" Working Draft, Standard for Programming Language C++">N4950</a>.
</p>

<ol>
<li><p>Modify 24.5.5.1 <a href="https://wg21.link/common.iterator">[common.iterator]</a>, class template <code>common_iterator</code> synopsis, as indicated:</p>


<blockquote>
<pre>
namespace std {
  template&lt;input_or_output_iterator I, sentinel_for&lt;I&gt; S&gt;
    requires (!same_as&lt;I, S&gt; &amp;&amp; copyable&lt;I&gt;)
  class common_iterator {
  public:
    [&hellip;]
    friend constexpr <del>iter_rvalue_reference_t&lt;I&gt;</del><ins>decltype(auto)</ins> iter_move(const common_iterator&amp; i)
      noexcept(noexcept(ranges::iter_move(declval&lt;const I&amp;&gt;())))
        requires input_iterator&lt;I&gt;;
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>

</li>

<li><p>Modify 24.5.5.7 <a href="https://wg21.link/common.iter.cust">[common.iter.cust]</a> as indicated:</p>

<blockquote>
<pre>
friend constexpr <del>iter_rvalue_reference_t&lt;I&gt;</del><ins>decltype(auto)</ins> iter_move(const common_iterator&amp; i)
  noexcept(noexcept(ranges::iter_move(declval&lt;const I&amp;&gt;())))
    requires input_iterator&lt;I&gt;;
</pre>
<blockquote>
<p>
-1- <i>Preconditions</i>: [&hellip;]
<p/>
-2- <i>Effects</i>: Equivalent to: <code>return ranges::iter_move(get&lt;I&gt;(i.v_));</code>
</p>
</blockquote>
</blockquote>

</li>

<li><p>Modify 24.5.7.1 <a href="https://wg21.link/counted.iterator">[counted.iterator]</a>, class template <code>counted_iterator</code> synopsis, as indicated:</p>


<blockquote>
<pre>
namespace std {
  template&lt;input_or_output_iterator I&gt;
  class counted_iterator {
  public:
    [&hellip;]
    friend constexpr <del>iter_rvalue_reference_t&lt;I&gt;</del><ins>decltype(auto)</ins> iter_move(const counted_iterator&amp; i)
      noexcept(noexcept(ranges::iter_move(i.current)))
        requires input_iterator&lt;I&gt;;
    [&hellip;]
  };
  [&hellip;]
}
</pre>
</blockquote>

</li>

<li><p>Modify 24.5.7.7 <a href="https://wg21.link/counted.iter.cust">[counted.iter.cust]</a> as indicated:</p>

<blockquote>
<pre>
friend constexpr <del>iter_rvalue_reference_t&lt;I&gt;</del><ins>decltype(auto)</ins> 
  iter_move(const counted_iterator&amp; i)
    noexcept(noexcept(ranges::iter_move(i.current)))
    requires input_iterator&lt;I&gt;;
</pre>
<blockquote>
<p>
-1- <i>Preconditions</i>: [&hellip;]
<p/>
-2- <i>Effects</i>: Equivalent to: <code>return ranges::iter_move(i.current);</code>
</p>
</blockquote>
</blockquote>

</li>

</ol>





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