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<title>Issue 3813: std::span&lt;volatile T, E&gt; is made ill-formed by P2278R4 when T is a normal class type</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#New">New</a> status.</em></p>
<h3 id="3813"><a href="lwg-active.html#3813">3813</a>. <code>std::span&lt;volatile T, E&gt;</code> is made ill-formed by P2278R4 when <code>T</code> is a normal class type</h3>
<p><b>Section:</b> 23.7.2.2.1 <a href="https://wg21.link/span.overview">[span.overview]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-11-06 <b>Last modified:</b> 2025-03-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#span.overview">issues</a> in [span.overview].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
This issue is discovered when implementing <a href="https://github.com/microsoft/STL/pull/3187#discussion_r1009941854">the 
<code>span</code> part of P2278R4 in MSVC STL</a>.
<p/>
<a href="https://wg21.link/P2278R4" title=" cbegin should always return a constant iterator">P2278R4</a> added the <code>const_iterator</code> member type to <code>std::span</code>, which required its 
<code>iterator</code> type to model <code>std::input_iterator</code> (same for <code>const_reverse_iterator</code> and 
<code>reverse_iterator</code>).
<p/>
However, when <code>element_type</code> is <code>volatile T</code> and <code>T</code> is a class type, the <code>iterator</code> 
type generally fails to satisfy <code>input_iterator</code>, because:
</p>
<ol>
<li><p><code>input_iterator&lt;iterator&gt;</code> requires <code>indirectly_readable&lt;iterator&gt;</code>;</p></li>
<li><p><code>indirectly_readable&lt;iterator&gt;</code> requires <code>common_reference_with&lt;iterator_reference_t&lt;iterator&gt;&amp;&amp;, 
iterator_rvalue_reference_t&lt;iterator&gt;&amp;&amp;&gt;</code>, that is 
<code>common_reference_with&lt;volatile T&amp;, volatile T&amp;&amp;&gt;</code>;</p></li>
<li><p><code>common_reference_t&lt;volatile T&amp;, volatile T&amp;&amp;&gt;</code> is <code>T</code> (which is problematic), 
and thus <code>common_reference_with&lt;volatile T&amp;, volatile T&amp;&amp;&gt;</code> requires both 
<code>convertible_to&lt;volatile T&amp;, T&gt;</code> and <code>convertible_to&lt;volatile T&amp;&amp;, T&gt;</code>;</p></li>
<li><p>However, the class type <code>T</code> generally doesn't have constructors from <code>volatile T</code> or 
<code>const volatile T</code> glvalues, and thus neither <code>convertible_to&lt;volatile T&amp;, T&gt;</code> and 
<code>convertible_to&lt;volatile T&amp;&amp;, T&gt;</code> is satisfied.</p></li>
</ol>
<p>
Ideally, <code>span</code> should not require any form of construction of <code>element_type</code>. Although usages of 
class types provided by the standard library via <code>volatile</code> glvalues are generally not supported, I think 
<code>span&lt;volatile T, E&gt;</code> is still useful for some user-defined class type <code>T</code>, and thus shouldn't be forbidden.
</p>

<p><i>[Kona 2022-11-12; Set priority to 2]</i></p>


<p><i>[2025-03-22; Jiang An comments and provides wording]</i></p>

<p>
The proposed resolution relaxes the <code class='backtick'>input_iterator</code> concept (which in turn relaxes stronger iterator concepts) 
and extends <code class='backtick'>iter_const_reference_t</code> and the exposition only <code><i>iter-const-rvalue-reference-t</i></code> 
aliases. Another approach can be just relaxing the <code class='backtick'>indirectly_readable</code> concept, but its impact may be larger 
than expected.
</p>


<p id="res-3813"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</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>
[&hellip;]
<i>// 24.3.4.9 <a href="https://wg21.link/iterator.concept.input">[iterator.concept.input]</a>, concept input_iterator</i>
<ins>template&lt;class I&gt;
  concept <i>deref-to-value-t</i> = <i>see below</i>;            <i>// freestanding</i>

template&lt;class I&gt;
  concept <i>weakly-indirectly-readable</i> = <i>see below</i>;  <i>// freestanding</i></ins>

template&lt;class I&gt;
concept input_iterator = <i>see below</i>;                <i>// freestanding</i>

[&hellip;]

<i>// 24.5.3.2 <a href="https://wg21.link/const.iterators.alias">[const.iterators.alias]</a>, alias templates</i>
template&lt;<del>indirectly_readable</del><ins><i>weakly-indirectly-readable</i></ins> I&gt;
  using iter_const_reference_t = <i>see below</i>;
</pre>
</blockquote>
</li>

<li><p>Modify 24.3.4.9 <a href="https://wg21.link/iterator.concept.input">[iterator.concept.input]</a>, concept <code>input_iterator</code> synopsis, as indicated:</p>

<blockquote>
<pre>
<ins>template&lt;class I&gt;
  concept <i>deref-to-value-t-impl</i> =      <i>// exposition only</i>
    same_as&lt;remove_cvref_t&lt;iter_reference_t&lt;I&gt;&gt;, iter_value_t&lt;I&gt;&gt; &amp;&amp;
    is_object_v&lt;iter_value_t&lt;I&gt;&gt;;

template&lt;class I&gt;
  concept <i>deref-to-value-t</i> =           <i>// exposition only</i>
    <i>deref-to-value-t-impl</i>&lt;remove_cvref_t&lt;I&gt;&gt;;

template&lt;class I&gt;
  concept <i>weakly-indirectly-readable</i> = <i>// exposition only</i>
    <i>deref-to-value-t</i>&lt;I&gt; ||
    indirectly_readable&lt;I&gt;;</ins>

template&lt;class I&gt;
  concept input_iterator =
    input_or_output_iterator&lt;I&gt; &amp;&amp;
    <del>indirectly_readable</del><ins><i>weakly-indirectly-readable</i></ins>&lt;I&gt; &amp;&amp;
    requires { typename <i>ITER_CONCEPT</i>(I); } &amp;&amp;
    derived_from&lt;<i>ITER_CONCEPT</i>(I), input_iterator_tag&gt;;
</pre>
</blockquote>
</li>

<li><p>Modify 24.5.3.2 <a href="https://wg21.link/const.iterators.alias">[const.iterators.alias]</a> as indicated:</p>

<blockquote>
<pre>
<ins>template&lt;class T&gt;
  using <i>ref-add-const-t</i> = <i>see below</i>;</ins>
</pre>
<blockquote>
<p>
<ins>-?- <i>Result</i>: If <code>is_lvalue_reference_v&lt;T&gt;</code> is <code class='backtick'>true</code>, 
<code>const remove_reference_t&lt;T&gt;&amp;</code>. Otherwise, if <code>is_rvalue_reference_v&lt;T&gt;</code> 
is <code class='backtick'>true</code>, <code>const remove_reference_t&lt;T&gt;&amp;&amp;</code>. Otherwise, <code>T</code>.</ins>
</p>
</blockquote>
<pre>
template&lt;<del>indirectly_readable</del><ins><i>weakly-indirectly-readable</i></ins> It&gt;
  using iter_const_reference_t = <ins>see below;</ins>
    <del>common_reference_t&lt;const iter_value_t&lt;It&gt;&amp;&amp;, iter_reference_t&lt;It&gt;&gt;;</del>
</pre>
<blockquote>
<p>
<ins>-?- <i>Result</i>: If <code class='backtick'>It</code> models <code><i>deref-to-value-t</i></code>, 
<code><i>ref-add-const-t</i>&lt;iter_reference_t&lt;It&gt;&gt;</code>. Otherwise, 
<code>common_reference_t&lt;const iter_value_t&lt;It&gt;&amp;&amp;, iter_reference_t&lt;It&gt;&gt;</code>.</ins>
</p>
</blockquote>
</blockquote>

<blockquote class="note">
<p>
[<i>Drafting note</i>: The simple <code><i>deref-to-value-t</i></code> case should be detected first, 
which avoids unnecessary instantiations and IFNDR-ness.]
</p>
</blockquote>

</li>

<li><p>Modify 24.5.3.3 <a href="https://wg21.link/const.iterators.iterator">[const.iterators.iterator]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;<del>indirectly_readable</del><ins><i>weakly-indirectly-readable</i></ins> I&gt;
  using <i>iter-const-rvalue-reference-t</i> =              <i>// exposition only</i>
    common_reference_t&lt;const iter_value_t&lt;I&gt;&amp;&amp;, iter_rvalue_reference_t&lt;I&gt;&gt;;
[&hellip;]
</pre>
</blockquote>
</li>

</ol>





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