<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3697: Preconditions of reference_constructs_from_temporary/reference_converts_from_temporary seem wrong</title>
<meta property="og:title" content="Issue 3697: Preconditions of reference_constructs_from_temporary/reference_converts_from_temporary seem wrong">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3697.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#New">New</a> status.</em></p>
<h3 id="3697"><a href="lwg-active.html#3697">3697</a>. Preconditions of <code>reference_constructs_from_temporary/reference_converts_from_temporary</code> seem wrong</h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-05-10 <b>Last modified:</b> 2022-05-17</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#meta.unary.prop">active issues</a> in [meta.unary.prop].</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.unary.prop">issues</a> in [meta.unary.prop].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::reference_constructs_from_temporary</code> and <code>std::reference_converts_from_temporary</code> are only 
useful when <code>T</code> is a reference type, and a reference type is always complete. Whenever <code>T</code> is an 
incomplete object type, it's clear that these traits inherit from <code>std::false_type</code>, without any further 
detection which may result in UB. However, when <code>T</code> is <code>X&amp;</code> or <code>X&amp;&amp;</code> where 
<code>X</code> is an incomplete object type, UB may be needed because of the potentially problematic detection in 
<code>std::is_constructible</code> or <code>std::is_convertible</code>.
<p/>
I'm not sure whether the general rule in 21.3.2 <a href="https://wg21.link/meta.rqmts">[meta.rqmts]</a>/5 affects these cases.
</p>

<p><i>[2022-05-17; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3697"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 21.3.3 <a href="https://wg21.link/meta.type.synop">[meta.type.synop]</a>, Table 46 ([tab:meta.unary.prop]) &mdash; 
"Type property predicates" &mdash; as indicated:</p>

<blockquote>
<table border="1">
<caption>Table 46: Type property predicates [tab:meta.unary.prop]</caption>
<tr>
<th align="center">Template</th>
<th align="center">Condition</th>
<th align="center">Preconditions</th>
</tr>

<tr>
<td colspan="3" align="center">
<code>&hellip;</code>
</td>
</tr>

<tr>
<td>
<code>template&lt;class T, class U&gt;<br/>
struct reference_constructs_from_temporary;</code>
</td>

<td>
<code>conjunction_v&lt;is_reference&lt;T&gt;, is_constructible&lt;T, U&gt;&gt;</code> is<br/> 
<code>true</code>, and the initialization <code>T t(<i>VAL</i>&lt;U&gt;);</code> binds <code>t</code> to a<br/> 
temporary object whose lifetime is extended (6.8.7 <a href="https://wg21.link/class.temporary">[class.temporary]</a>).
</td>

<td>
<ins>If <code>T</code> is a reference type, <code>remove_reference_t&lt;T&gt;</code><br/> 
and <code>U</code></ins><del><code>T</code></del> shall be <del>a</del> complete type<ins>s</ins>,<br/>
<i>cv</i> <code>void</code>, or <del>an</del> array<ins>s</ins> of unknown bound.
</td>
</tr>

<tr>
<td>
<code>template&lt;class T, class U&gt;<br/>
struct reference_converts_from_temporary;</code>
</td>

<td>
<code>conjunction_v&lt;is_reference&lt;T&gt;, is_convertible&lt;U, T&gt;&gt;</code> is<br/> 
<code>true</code>, and the initialization <code>T t = <i>VAL</i>&lt;U&gt;;</code> binds <code>t</code> to a<br/> 
temporary object whose lifetime is extended (6.8.7 <a href="https://wg21.link/class.temporary">[class.temporary]</a>).
</td>

<td>
<ins>If</ins> <code>T</code> <ins>is a reference type, <code>remove_reference_t&lt;T&gt;</code></ins><br/> 
and <code>U</code> shall be complete types,<br/>
<i>cv</i> <code>void</code>, or arrays of unknown bound.
</td>
</tr>

</table>
</blockquote>

</li>
</ol>





</body>
</html>
