<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 2933: PR for LWG 2773 could be clearer</title>
<meta property="og:title" content="Issue 2933: PR for LWG 2773 could be clearer">
<meta property="og:description" content="C++ library issue. Status: Resolved">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue2933.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="2933"><a href="lwg-defects.html#2933">2933</a>. PR for LWG 2773 could be clearer</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> Eric Fiselier <b>Opened:</b> 2017-02-06 <b>Last modified:</b> 2024-07-25</p>
<p><b>Priority: </b>3
</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>
The current PR for LWG <a href="lwg-defects.html#2773" title="Making std::ignore constexpr (Status: C++17)">2773</a><sup><a href="https://cplusplus.github.io/LWG/issue2773" title="Latest snapshot">(i)</a></sup> changes <code>std::ignore</code> to be a <code>constexpr</code> variable. However it says 
nothing about whether using <code>std::ignore</code> in <code>std::tie</code> is a constant expression. I think the intent was clearly 
to allow this. Therefore I suggest we update the resolution to explicitly call this out in a note. (I don't think new normative 
wording is needed).
<p/>
I propose we update the current PR as follows:
</p>
<ol>
<li><p>Keep the current changes proposed by the PR.</p></li>
<li><p>Add a note after [tuple.creation]/p7 (<code>std::tie</code>):</p>
<blockquote><p>
<ins>[<i>Note:</i> The constructors and assignment operators provided by <code>ignore</code> shall be <code>constexpr</code>]</ins>
</p></blockquote>
</li>
</ol>
<p>
Perhaps LWG feels the existing wording is clear enough, but if not I think the above changes sufficiently clarify it.
<p/>
The ability to <code>constexpr</code> assign to <code>std::ignore</code> can be important: Here is an extremely contrived example:
</p>
<blockquote><pre>
constexpr bool foo() {
  auto res = std::tie(std::ignore);
  std::get&lt;0&gt;(res) =42; 
  return true;
}
static_assert(foo());
</pre></blockquote>
<p><i>[2017-03-04, Kona]</i></p>

<p>Set priority to 3. P/R is incorrect; it should NOT be a note. Marshall to work with Eric to get better wording. STL says "use an exposition-only class".</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-2933"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Modify 22.4.5 <a href="https://wg21.link/tuple.creation">[tuple.creation]</a> as indicated:</p>
<blockquote><pre>
template&lt;class... TTypes&gt;
  constexpr tuple&lt;TTypes&amp;...> tie(TTypes&amp;... t) noexcept;
</pre>
<blockquote>
<p>
-7- <i>Returns:</i> [&hellip;]
<p/>
<ins>-?- [<i>Note:</i> The constructors and assignment operators provided by <code>ignore</code> shall be <code>constexpr</code>. &mdash; <i>end note</i>]</ins>
<p/>
-8- [<i>Example:</i> [&hellip;] &mdash; <i>end example</i>]
</p>
</blockquote>
</blockquote>
</li>

</ol>






</body>
</html>
