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<title>Issue 3643: Missing constexpr in std::counted_iterator</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="3643"><a href="lwg-defects.html#3643">3643</a>. Missing <code>constexpr</code> in <code>std::counted_iterator</code></h3>
<p><b>Section:</b> 24.5.7.5 <a href="https://wg21.link/counted.iter.nav">[counted.iter.nav]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2021-11-21 <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#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
One overload of <code>std::counted_operator::operator++</code> is not <code>constexpr</code> currently, 
which is seemly because of that a try-block (specified in 24.5.7.5 <a href="https://wg21.link/counted.iter.nav">[counted.iter.nav]</a>/4) 
is not allowed in a constexpr function until C++20. Given a try-block is allowed in a constexpr 
function in C++20, IMO this overload should also be <code>constexpr</code>.
<p/>
MSVC STL has already added <code>constexpr</code> <a href="https://github.com/microsoft/STL/blob/1e42166b78f814c9150afff98cc399773eda8ddf/stl/inc/iterator#L599">at first</a>. 
The situation of this overload is originally found by Casey Carter, but no LWG issue has been submitted.
</p>

<p><i>[2022-01-30; Reflector poll]</i></p>

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

<p><i>[2022-02-10 Approved at February 2022 virtual plenary. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3643"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4901" title=" Working Draft, Standard for Programming Language C++">N4901</a>.
</p>

<ol>

<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>
[&hellip;]
constexpr counted_iterator&amp; operator++();
<ins>constexpr</ins> decltype(auto) operator++(int);
constexpr counted_iterator operator++(int)
  requires forward_iterator&lt;I&gt;;
constexpr counted_iterator&amp; operator--()
  requires bidirectional_iterator&lt;I&gt;;
constexpr counted_iterator operator--(int)
  requires bidirectional_iterator&lt;I&gt;;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.7.5 <a href="https://wg21.link/counted.iter.nav">[counted.iter.nav]</a> as indicated:</p>

<blockquote>
<pre>
<ins>constexpr</ins> decltype(auto) operator++(int);
</pre>
<blockquote>
<p>
-3- <i>Preconditions:</i> <code>length &gt; 0</code>.
<p/>
-4- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
--length;
try { return current++; }
catch(...) { ++length; throw; }
</pre></blockquote>
</blockquote>
</blockquote>
</li>

</ol>





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