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<title>Issue 3574: common_iterator should be completely constexpr-able</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="3574"><a href="lwg-defects.html#3574">3574</a>. <code>common_iterator</code> should be completely <code>constexpr</code>-able</h3>
<p><b>Section:</b> 24.5.5.1 <a href="https://wg21.link/common.iterator">[common.iterator]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2021-07-21 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#common.iterator">active issues</a> in [common.iterator].</p>
<p><b>View all other</b> <a href="lwg-index.html#common.iterator">issues</a> in [common.iterator].</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>
After <a href="https://wg21.link/P2231R1" title=" Add further constexpr support for optional/variant">P2231R1</a>, <code>variant</code> becomes completely <code>constexpr</code>-able, which means that 
<code>common_iterator</code> can also be completely <code>constexpr</code>-able.
</p>


<p><i>[2021-08-20; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>


<p><i>[2021-08-23; 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-3574"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</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:
    constexpr common_iterator() requires default_initializable&lt;I&gt; = default;
    constexpr common_iterator(I i);
    constexpr common_iterator(S s);
    template&lt;class I2, class S2&gt;
      requires convertible_to&lt;const I2&amp;, I&gt; &amp;&amp; convertible_to&lt;const S2&amp;, S&gt;
        constexpr common_iterator(const common_iterator&lt;I2, S2&gt;&amp; x);
        
    template&lt;class I2, class S2&gt;
      requires convertible_to&lt;const I2&amp;, I&gt; &amp;&amp; convertible_to&lt;const S2&amp;, S&gt; &amp;&amp;
               assignable_from&lt;I&amp;, const I2&amp;&gt; &amp;&amp; assignable_from&lt;S&amp;, const S2&amp;&gt;
      <ins>constexpr</ins> common_iterator&amp; operator=(const common_iterator&lt;I2, S2&gt;&amp; x);

    <ins>constexpr</ins> decltype(auto) operator*();
    <ins>constexpr</ins> decltype(auto) operator*() const
      requires <i>dereferenceable</i>&lt;const I&gt;;
    <ins>constexpr</ins> decltype(auto) operator-&gt;() const
      requires <i>see below</i>;
 
    <ins>constexpr</ins> common_iterator&amp; operator++();
    <ins>constexpr</ins> decltype(auto) operator++(int);

    template&lt;class I2, sentinel_for&lt;I&gt; S2&gt;
      requires sentinel_for&lt;S, I2&gt;
    friend <ins>constexpr</ins> bool operator==(
      const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);
    template&lt;class I2, sentinel_for&lt;I&gt; S2&gt;
      requires sentinel_for&lt;S, I2&gt; &amp;&amp; equality_comparable_with&lt;I, I2&gt;
    friend <ins>constexpr</ins> bool operator==(
      const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);

    template&lt;sized_sentinel_for&lt;I&gt; I2, sized_sentinel_for&lt;I&gt; S2&gt;
      requires sized_sentinel_for&lt;S, I2&gt;
    friend <ins>constexpr</ins> iter_difference_t&lt;I2&gt; operator-(
      const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);

    friend <ins>constexpr</ins> iter_rvalue_reference_t&lt;I&gt; iter_move(const common_iterator&amp; i)
      noexcept(noexcept(ranges::iter_move(declval&lt;const I&amp;&gt;())))
        requires input_iterator&lt;I&gt;;
    template&lt;indirectly_swappable&lt;I&gt; I2, class S2&gt;
      friend <ins>constexpr</ins> void iter_swap(const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y)
        noexcept(noexcept(ranges::iter_swap(declval&lt;const I&amp;&gt;(), declval&lt;const I2&amp;&gt;())));

  private:
    variant&lt;I, S&gt; v_; <i>// exposition only</i>
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.5.3 <a href="https://wg21.link/common.iter.const">[common.iter.const]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class I2, class S2&gt;
  requires convertible_to&lt;const I2&amp;, I&gt; &amp;&amp; convertible_to&lt;const S2&amp;, S&gt; &amp;&amp;
           assignable_from&lt;I&amp;, const I2&amp;&gt; &amp;&amp; assignable_from&lt;S&amp;, const S2&amp;&gt;
    <ins>constexpr</ins> common_iterator&amp; operator=(const common_iterator&lt;I2, S2&gt;&amp; x);
</pre>
<blockquote>
<p>
-5- <i>Preconditions:</i> <code>x.v_.valueless_by_exception()</code> is <code>false</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.5.5.4 <a href="https://wg21.link/common.iter.access">[common.iter.access]</a> as indicated:</p>

<blockquote>
<pre>
<ins>constexpr</ins> decltype(auto) operator*();
<ins>constexpr</ins> decltype(auto) operator*() const
  requires <i>dereferenceable</i>&lt;const I&gt;;
</pre>
<blockquote>
<p>
-1- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
<ins>constexpr</ins> decltype(auto) operator-&gt;() const
  requires <i>see below</i>;
</pre>
<blockquote>
<p>
-3- The expression in the <i>requires-clause</i> is equivalent to:
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.5.5.5 <a href="https://wg21.link/common.iter.nav">[common.iter.nav]</a> as indicated:</p>

<blockquote>
<pre>
<ins>constexpr</ins> common_iterator&amp; operator++();
</pre>
<blockquote>
<p>
-1- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
<ins>constexpr</ins> decltype(auto) operator++(int);
</pre>
<blockquote>
<p>
-4- <i>Preconditions:</i> <code>holds_alternative&lt;I&gt;(v_)</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 24.5.5.6 <a href="https://wg21.link/common.iter.cmp">[common.iter.cmp]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class I2, sentinel_for&lt;I&gt; S2&gt;
  requires sentinel_for&lt;S, I2&gt;
friend <ins>constexpr</ins> bool operator==(
  const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);
</pre>
<blockquote>
<p>
-1- <i>Preconditions:</i> [&hellip;]
</p>
</blockquote>
<pre>
template&lt;class I2, sentinel_for&lt;I&gt; S2&gt;
  requires sentinel_for&lt;S, I2&gt; &amp;&amp; equality_comparable_with&lt;I, I2&gt;
friend <ins>constexpr</ins> bool operator==(
  const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);
</pre>
<blockquote>
<p>
-3- <i>Preconditions:</i> [&hellip;]
</p>
</blockquote>
<pre>
template&lt;sized_sentinel_for&lt;I&gt; I2, sized_sentinel_for&lt;I&gt; S2&gt;
  requires sized_sentinel_for&lt;S, I2&gt;
friend <ins>constexpr</ins> iter_difference_t&lt;I2&gt; operator-(
  const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y);
</pre>
<blockquote>
<p>
-5- <i>Preconditions:</i> [&hellip;]
</p>
</blockquote>
</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 <ins>constexpr</ins> iter_rvalue_reference_t&lt;I&gt; 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>
</blockquote>
<pre>
template&lt;indirectly_swappable&lt;I&gt; I2, class S2&gt;
  friend <ins>constexpr</ins> void iter_swap(const common_iterator&amp; x, const common_iterator&lt;I2, S2&gt;&amp; y)
    noexcept(noexcept(ranges::iter_swap(declval&lt;const I&amp;&gt;(), declval&lt;const I2&amp;&gt;())));
</pre>
<blockquote>
<p>
-3- <i>Preconditions:</i> [&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>




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