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<title>Issue 3853: basic_const_iterator&lt;volatile int*&gt;::operator-&gt; is ill-formed</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="3853"><a href="lwg-defects.html#3853">3853</a>. <code>basic_const_iterator&lt;volatile int*&gt;::operator-&gt;</code> is ill-formed</h3>
<p><b>Section:</b> 24.5.3 <a href="https://wg21.link/const.iterators">[const.iterators]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2023-01-06 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#const.iterators">active issues</a> in [const.iterators].</p>
<p><b>View all other</b> <a href="lwg-index.html#const.iterators">issues</a> in [const.iterators].</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>
Currently, <code>basic_const_iterator::operator-&gt;</code> constrains the value type
of the underlying iterator to be only the <i>cv</i>-unqualified type of its reference type, 
which is true for raw pointers.
</p>
<p>
However, since it also explicitly specifies returning a pointer to a <code>const</code> value type,
this will cause a hard error when the value type is actually <code>volatile</code>-qualified:
</p>
<blockquote>
<pre>
std::basic_const_iterator&lt;volatile int*&gt; it;
auto* p = it.operator-&gt;(); // <span style="color:red;font-weight:bolder">invalid conversion from 'volatile int*' to 'const int*'</span>
</pre>
</blockquote>
<p>
The proposed resolution changes the return type from <code>const value_type*</code>
to <code>const auto*</code>,
which makes it deduce the correct type in the above example,
i.e. <code>const volatile int*</code>.
</p>

<p><i>[2023-02-01; Reflector poll]</i></p>

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

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



<p id="res-3853"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Modify 24.5.3 <a href="https://wg21.link/const.iterators">[const.iterators]</a> as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class I&gt;
    concept <i>not-a-const-iterator</i> = <i>see below</i>;

  template&lt;input_iterator Iterator&gt;
  class basic_const_iterator {
    Iterator <i>current_</i> = Iterator();                             // <i>exposition only</i>
    using <i>reference</i> = iter_const_reference_t&lt;Iterator&gt;;         // <i>exposition only</i>
  
  public:
    [&hellip;]
    constexpr <i>reference</i> operator*() const;
    constexpr const <ins>auto</ins><del>value_type</del>* operator-&gt;() const
       requires is_lvalue_reference_v&lt;iter_reference_t&lt;Iterator&gt;&gt; &amp;&amp;
                same_as&lt;remove_cvref_t&lt;iter_reference_t&lt;Iterator&gt;&gt;, value_type&gt;;
    [&hellip;]
  };
}
</pre>
[&hellip;]
<pre>
constexpr const <ins>auto</ins><del>value_type</del>* operator-&gt;() const
  requires is_lvalue_reference_v&lt;iter_reference_t&lt;Iterator&gt;&gt; &amp;&amp;
           same_as&lt;remove_cvref_t&lt;iter_reference_t&lt;Iterator&gt;&gt;, value_type&gt;;
</pre>
<p>
-7- <i>Returns</i>: If <code>Iterator</code> models <code>contiguous_iterator</code>, <code>to_address(<i>current_</i>)</code>; otherwise, 
<code>addressof(*<i>current_</i>)</code>.
</p>
</blockquote>
</li>
</ol>






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