<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4186: regex_traits::transform_primary mistakenly detects typeid of a function</title>
<meta property="og:title" content="Issue 4186: regex_traits::transform_primary mistakenly detects typeid of a function">
<meta property="og:description" content="C++ library issue. Status: WP">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4186.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#WP">WP</a> status.</em></p>
<h3 id="4186"><a href="lwg-defects.html#4186">4186</a>. <code class='backtick'>regex_traits::transform_primary</code> mistakenly detects <code class='backtick'>typeid</code> of a function</h3>
<p><b>Section:</b> 28.6.6 <a href="https://wg21.link/re.traits">[re.traits]</a> <b>Status:</b> <a href="lwg-active.html#WP">WP</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2024-12-18 <b>Last modified:</b> 2025-02-16</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#re.traits">issues</a> in [re.traits].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#WP">WP</a> status.</p>
<p><b>Discussion:</b></p>
<p>
28.6.6 <a href="https://wg21.link/re.traits">[re.traits]</a>/7 currently says <code>typeid(use_facet&lt;collate&lt;charT&gt;&gt;) == 
typeid(collate_byname&lt;charT&gt;)</code>, which is always <code class='backtick'>false</code> because 
<code>use_facet&lt;collate&lt;charT&gt;&gt;</code> is a function template specialization while 
<code>collate_byname&lt;charT&gt;</code> is a class template specialization. This looks like 
misuse, and has been present in TR1 (<a href="https://wg21.link/N1836" title=" Draft Technical Report on C++ Library Extensions">N1836</a>).
<p/>
Presumably the intended operand is <code>use_facet&lt;collate&lt;charT&gt;&gt;(getloc())</code>.
</p>

<p><i>[2025-02-07; Reflector poll]</i></p>

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

<p><i>[Hagenberg 2025-02-16; Status changed: Voting &rarr; WP.]</i></p>



<p id="res-4186"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5001" title=" Working Draft, Programming Languages — C++">N5001</a>.
</p>

<ol>
<li><p>Modify 28.6.6 <a href="https://wg21.link/re.traits">[re.traits]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class ForwardIterator&gt;
  string_type transform_primary(ForwardIterator first, ForwardIterator last) const;
</pre>
<blockquote>
<p>
-7- <i>Effects</i>: If
</p>
<blockquote><pre>
typeid(use_facet&lt;collate&lt;charT&gt;&gt;<ins>(getloc())</ins>) == typeid(collate_byname&lt;charT&gt;)
</pre></blockquote>
<p>
and the form of the sort key returned by <code>collate_byname&lt;charT&gt;::transform(first, last)</code> 
is known and can be converted into a primary sort key then returns that key, otherwise returns an empty
string.
</p>
</blockquote>
</blockquote>


</li>

</ol>





</body>
</html>
