<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 3978: The "no effect" requirement for std::ignore is unimplementable for volatile bit-fields</title>
<meta property="og:title" content="Issue 3978: The &quot;no effect&quot; requirement for std::ignore is unimplementable for volatile bit-fields">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue3978.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#Resolved">Resolved</a> status.</em></p>
<h3 id="3978"><a href="lwg-defects.html#3978">3978</a>. The "no effect" requirement for <code>std::ignore</code> is unimplementable for <code>volatile</code> bit-fields</h3>
<p><b>Section:</b> 22.4.5 <a href="https://wg21.link/tuple.creation">[tuple.creation]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-08-19 <b>Last modified:</b> 2024-07-25</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#tuple.creation">issues</a> in [tuple.creation].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
22.4.5 <a href="https://wg21.link/tuple.creation">[tuple.creation]</a>/5 currently says:
</p>
<blockquote style="border-left: 3px solid #ccc;padding-left: 15px;"><p>
[&hellip;] When an argument in <code>t</code> is <code>ignore</code>, assigning any value to the corresponding tuple element has no effect.
</p></blockquote>
<p>
which is unimplementable for <code>volatile</code>-qualified bit-field glvalues.
<p/>
In order to passing such a value to the <code>operator=</code> function, a distinct object needs to be created and thus the read from 
the <code>volatile</code> glvalue, which is a side effect (6.10.1 <a href="https://wg21.link/intro.execution">[intro.execution]</a>/7), is unavoidable.
<p/>
<a href="https://wg21.link/P2968R0" title=" Make std::ignore a first-class object">P2968R0</a> addresses the impossibility of assignment from <code>void</code> values, but doesn't talk about 
<code>volatile</code> bit-fields. Perhaps we should explicitly say that the program is ill-formed if a <code>volatile</code> bit-field 
value is assigned to <code>std::ignore</code> (which is implemented in libstdc++ and MSVC STL, but not in libc++).
</p>

<p><i>[2023-11-03; Reflector poll]</i></p>

<p>
Set priority to 4 after reflector poll.
"Specify it as code."
"<a href="https://wg21.link/P2968" title=" Make std::ignore a first-class object">P2968</a> should fix this."
</p>

<p><i>[2024-07-25 Status changed: New &rarr; Resolved.]</i></p>

<p>
Resolved by <a href="https://wg21.link/P2968R2" title=" Make std::ignore a first-class object">P2968R2</a>, approved in St. Louis.
</p>


<p id="res-3978"><b>Proposed resolution:</b></p>





</body>
</html>
