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<title>Issue 3446: indirectly_readable_traits ambiguity for types with both value_type and element_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#C++23">C++23</a> status.</em></p>
<h3 id="3446"><a href="lwg-defects.html#3446">3446</a>. <code>indirectly_readable_traits</code> ambiguity for types with both <code>value_type</code> and <code>element_type</code></h3>
<p><b>Section:</b> 24.3.2.2 <a href="https://wg21.link/readable.traits">[readable.traits]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-05-15 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#readable.traits">issues</a> in [readable.traits].</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>
Per 24.3.2.2 <a href="https://wg21.link/readable.traits">[readable.traits]</a>, <code>indirectly_readable_traits&lt;T&gt;::value_type</code> is the same type as
<code>remove_cv_t&lt;T::value_type&gt;</code> if it denotes an object type, or <code>remove_cv_t&lt;T::element_type&gt;</code>
if it denotes an object type. If both <code>T::value_type</code> and <code>T::element_type</code> denote types,
<code>indirectly_readable_traits&lt;T&gt;::value_type</code> is ill-formed. This was perhaps not the best design,
given that there are iterators in the wild (Boost's unordered containers) that define both nested types.
<code>indirectly_readable_traits</code> should tolerate iterators that define both nested types consistently.
</p>

<p><i>[2020-07-17; Priority set to 2 in telecon]</i></p>


<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

<ol>
<li><p>Modify 24.3.2.2 <a href="https://wg21.link/readable.traits">[readable.traits]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;class&gt; struct <i>cond-value-type</i> { }; <i>// exposition only</i>

template&lt;class T&gt;
  requires is_object_v&lt;T&gt;
struct <i>cond-value-type</i>&lt;T&gt; {
  using value_type = remove_cv_t&lt;T&gt;;
};

template&lt;class&gt; struct indirectly_readable_traits { };

[&hellip;]

template&lt;class T&gt;
  requires requires { typename T::value_type; }
struct indirectly_readable_traits&lt;T&gt;
  : <i>cond-value-type</i>&lt;typename T::value_type&gt; { };

template&lt;class T&gt;
  requires requires { typename T::element_type; }
struct indirectly_readable_traits&lt;T&gt;
  : <i>cond-value-type</i>&lt;typename T::element_type&gt; { };

<ins>template&lt;class T&gt;</ins>
  <ins>requires requires {</ins>
    <ins>typename T::element_type;</ins>
    <ins>typename T::value_type;</ins>
    <ins>requires same_as&lt;</ins>
      <ins>remove_cv_t&lt;typename T::element_type&gt;,</ins>
      <ins>remove_cv_t&lt;typename T::value_type&gt;&gt;;</ins>
  <ins>}</ins>
<ins>struct indirectly_readable_traits&lt;T&gt;</ins>
  <ins>: <i>cond-value-type</i>&lt;typename T::value_type&gt; { };</ins>

[&hellip;]
</pre>
</blockquote>
</li>

</ol>
</blockquote>

<p><i>[2020-07-23; Casey improves wording per reflector discussion]</i></p>


<p><i>[2020-08-21; moved to Tentatively Ready after five votes in favour in reflector poll]</i></p>


<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3446"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

<ol>
<li><p>Modify 24.3.2.2 <a href="https://wg21.link/readable.traits">[readable.traits]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;class&gt; struct <i>cond-value-type</i> { }; <i>// exposition only</i>

template&lt;class T&gt;
  requires is_object_v&lt;T&gt;
struct <i>cond-value-type</i>&lt;T&gt; {
  using value_type = remove_cv_t&lt;T&gt;;
};

<ins>template&lt;class T&gt;</ins>
<ins>concept <i>has-member-value-type</i> = requires { typename T::value_type; }; <i>// exposition only</i></ins>

<ins>template&lt;class T&gt;</ins>
<ins>concept <i>has-member-element-type</i> = requires { typename T::element_type; }; <i>// exposition only</i></ins>

template&lt;class&gt; struct indirectly_readable_traits { };

[&hellip;]

template&lt;<del>class</del><ins><i>has-member-value-type</i></ins> T&gt;
  <del>requires requires { typename T::value_type; }</del>
struct indirectly_readable_traits&lt;T&gt;
  : <i>cond-value-type</i>&lt;typename T::value_type&gt; { };

template&lt;<del>class</del><ins><i>has-member-element-type</i></ins> T&gt;
  <del>requires requires { typename T::element_type; }</del>
struct indirectly_readable_traits&lt;T&gt;
  : <i>cond-value-type</i>&lt;typename T::element_type&gt; { };

<ins>template&lt;<del>class</del><ins><i>has-member-value-type</i></ins> T&gt;</ins>
  <ins>requires <i>has-member-element-type</i>&lt;T&gt; &amp;&amp;</ins>
           <ins>same_as&lt;remove_cv_t&lt;typename T::element_type&gt;, remove_cv_t&lt;typename T::value_type&gt;&gt;</ins>
<ins>struct indirectly_readable_traits&lt;T&gt;</ins>
  <ins>: <i>cond-value-type</i>&lt;typename T::value_type&gt; { };</ins>

[&hellip;]
</pre>
</blockquote>
</li>

</ol>




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