<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3541: indirectly_readable_traits should be SFINAE-friendly for all types</title>
<meta property="og:title" content="Issue 3541: indirectly_readable_traits should be SFINAE-friendly for all types">
<meta property="og:description" content="C++ library issue. Status: C++23">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3541.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="3541"><a href="lwg-defects.html#3541">3541</a>. <code>indirectly_readable_traits</code> should be SFINAE-friendly for all types</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> Christopher Di Bella <b>Opened:</b> 2021-04-08 <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#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>
Following the outcome of LWG <a href="lwg-defects.html#3446" title="indirectly_readable_traits ambiguity for types with both value_type and element_type (Status: C++23)">3446</a><sup><a href="https://cplusplus.github.io/LWG/issue3446" title="Latest snapshot">(i)</a></sup>, a strict implementation of <code>std::indirectly_readable_traits</code> 
isn't SFINAE-friendly for types that have different <code>value_type</code> and <code>element_type</code> members due to 
the ambiguity between the <code><i>has-member-value-type</i></code> and <code><i>has-member-element-type</i></code> 
specialisations. Other traits types are empty when requirements aren't met; we should follow suit here.
</p>

<p><i>[2021-04-20; Reflector poll]</i></p>

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

<p><i>[2021-06-07 Approved at June 2021 virtual plenary. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3541"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4885">N4885</a>.
</p>

<ol>
<li><p>Modify 24.3.2.2 <a href="https://wg21.link/readable.traits">[readable.traits]</a> p1 as indicated:</p>

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

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

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

[&hellip;]

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

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

<ins>template&lt;<i>has-member-value-type</i> T&gt;
  requires <i>has-member-element-type</i>&lt;T&gt;
struct indirectly_readable_traits&lt;T&gt; { };</ins>
  
template&lt;<i>has-member-value-type</i> T&gt;
  requires <i>has-member-element-type</i>&lt;T&gt; &amp;&amp;
           same_as&lt;remove_cv_t&lt;typename T::element_type&gt;, remove_cv_t&lt;typename T::value_type&gt;&gt;
struct indirectly_readable_traits&lt;T&gt;
  : <i>cond-value-type</i>&lt;typename T::value_type&gt; { };
[&hellip;]
</pre>
</blockquote>
</li>

</ol>




</body>
</html>
