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<title>Issue 3862: basic_const_iterator's common_type specialization is underconstrained</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="3862"><a href="lwg-defects.html#3862">3862</a>. <code>basic_const_iterator</code>'s <code>common_type</code> specialization is underconstrained</h3>
<p><b>Section:</b> 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-01-25 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#iterator.synopsis">issues</a> in [iterator.synopsis].</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>
To make <code>basic_const_iterator</code> compatible with its unwrapped iterators, the standard defines the following
<code>common_type</code> specialization:
</p>
<blockquote><pre>
template&lt;class T, common_with&lt;T&gt; U&gt;
struct common_type&lt;basic_const_iterator&lt;T&gt;, U&gt; {
  using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
};
</pre></blockquote>
<p>
For type <code>U</code>, when it shares a common type with the unwrapped type <code>T</code> of <code>basic_const_iterator</code>,
the common type of both is <code>basic_const_iterator</code> of the common type of <code>T</code> and <code>U</code>.
<p/>
However, since this specialization only constrains <code>U</code> and <code>T</code> to have a common type,
this allows <code>U</code> to be any type that satisfies such requirement, such as <code>optional&lt;T&gt;</code>,
in which case computing the common type of both would produce a hard error inside the specialization,
because <code>basic_const_iterator</code> requires the template parameter to be <code>input_iterator</code>,
while <code>optional</code> clearly isn't.
</p>
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt;              // <i>see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>
#include &lt;concepts&gt;             // <i>see 18.3 <a href="https://wg21.link/concepts.syn">[concepts.syn]</a></i>

namespace std {
  [&hellip;]
  template&lt;class T, common_with&lt;T&gt; U&gt;
    <ins>requires input_iterator&lt;U&gt;</ins>
  struct common_type&lt;basic_const_iterator&lt;T&gt;, U&gt; {                                  // <i>freestanding</i>
    using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
  };
  template&lt;class T, common_with&lt;T&gt; U&gt;
    <ins>requires input_iterator&lt;U&gt;</ins>
  struct common_type&lt;U, basic_const_iterator&lt;T&gt;&gt; {                                  // <i>freestanding</i>
    using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2023-02-06; Jonathan provides improved wording based on Casey's
suggestion during the prioritization poll.]</i></p>


<p><i>[Issaquah 2023-02-07; LWG]</i></p>

<p>Move to Immediate for C++23</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3862"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 24.2 <a href="https://wg21.link/iterator.synopsis">[iterator.synopsis]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
#include &lt;compare&gt;              // <i>see 17.12.1 <a href="https://wg21.link/compare.syn">[compare.syn]</a></i>
#include &lt;concepts&gt;             // <i>see 18.3 <a href="https://wg21.link/concepts.syn">[concepts.syn]</a></i>

namespace std {
  [&hellip;]
  template&lt;class T, common_with&lt;T&gt; U&gt;
    <ins>requires input_iterator&lt;common_type_t&lt;T, U&gt;&gt;</ins>
  struct common_type&lt;basic_const_iterator&lt;T&gt;, U&gt; {                                  // <i>freestanding</i>
    using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
  };
  template&lt;class T, common_with&lt;T&gt; U&gt;
    <ins>requires input_iterator&lt;common_type_t&lt;T, U&gt;&gt;</ins>
  struct common_type&lt;U, basic_const_iterator&lt;T&gt;&gt; {                                  // <i>freestanding</i>
    using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
  };
  template&lt;class T, common_with&lt;T&gt; U&gt;
    <ins>requires input_iterator&lt;common_type_t&lt;T, U&gt;&gt;</ins>
  struct common_type&lt;basic_const_iterator&lt;T&gt;, basic_const_iterator&lt;U&gt;&gt; {            // <i>freestanding</i>
    using type = basic_const_iterator&lt;common_type_t&lt;T, U&gt;&gt;;
  };
  [&hellip;]
}
</pre>
</blockquote>
</li>

</ol>





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