<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4026: Assignment operators of std::expected should propagate triviality</title>
<meta property="og:title" content="Issue 4026: Assignment operators of std::expected should propagate triviality">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4026.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="4026"><a href="lwg-active.html#4026">4026</a>. Assignment operators of <code>std::expected</code> should propagate triviality</h3>
<p><b>Section:</b> 22.8.6.4 <a href="https://wg21.link/expected.object.assign">[expected.object.assign]</a>, 22.8.7.4 <a href="https://wg21.link/expected.void.assign">[expected.void.assign]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-12-16 <b>Last modified:</b> 2024-03-11</p>
<p><b>Priority: </b>2
</p>
<p><b>View other</b> <a href="lwg-index-open.html#expected.object.assign">active issues</a> in [expected.object.assign].</p>
<p><b>View all other</b> <a href="lwg-index.html#expected.object.assign">issues</a> in [expected.object.assign].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, only copy and move constructors of <code>std::expected</code> are required to propagate triviality, 
while copy and move assignment operators are not. Given that the assignment operators of <code>std::optional</code> 
and <code>std::variant</code> are already required to propagate triviality, it seems to me that we should also 
apply such requirements for <code>std::expected</code>.
<p/>
Such changes are being made in libc++ (<a href="https://github.com/llvm/llvm-project/pull/74768">llvm/llvm-project#74768</a>). 
And it may be desired to make the triviality improvement portable.
</p>

<p><i>[2024-03-11; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll in January 2024.
A few votes for Tentatively Ready, others thought it needed more consideration.
</p>



<p id="res-4026"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>

<li><p>Modify 22.8.6.4 <a href="https://wg21.link/expected.object.assign">[expected.object.assign]</a> as indicated:</p>

<blockquote>
<pre>
constexpr expected&amp; operator=(const expected&amp; rhs);
</pre>
<blockquote>
<p>
-2- <i>Effects</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-4- <i>Remarks</i>: This operator is defined as deleted unless:
</p>
<ol style="list-style-type: none">
<li><p>[&hellip;]</p></li>
</ol>
<p>
<ins>-?- This operator is trivial if:</ins>
</p>
<ol style="list-style-type: none">
<li><p><ins>(?.1) &mdash; <code>is_trivially_copy_constructible_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.2) &mdash; <code>is_trivially_copy_assignable_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.3) &mdash; <code>is_trivially_destructible_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.4) &mdash; <code>is_trivially_copy_constructible_v&lt;E&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.5) &mdash; <code>is_trivially_copy_assignable_v&lt;E&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.6) &mdash; <code>is_trivially_destructible_v&lt;E&gt;</code> is <code>true</code>.</ins></p></li>
</ol>
</blockquote>
<pre>
constexpr expected&amp; operator=(expected&amp;&amp; rhs) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-5- <i>Constraints</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-8- <i>Remarks</i>: The exception specification is equivalent to:
<p/>
[&hellip;]
</p>
<p>
<ins>-?- This operator is trivial if:</ins>
</p>
<ol style="list-style-type: none">
<li><p><ins>(?.1) &mdash; <code>is_trivially_move_constructible_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.2) &mdash; <code>is_trivially_move_assignable_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.3) &mdash; <code>is_trivially_destructible_v&lt;T&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.4) &mdash; <code>is_trivially_move_constructible_v&lt;E&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.5) &mdash; <code>is_trivially_move_assignable_v&lt;E&gt;</code> is <code>true</code>, and</ins></p></li>
<li><p><ins>(?.6) &mdash; <code>is_trivially_destructible_v&lt;E&gt;</code> is <code>true</code>.</ins></p></li>
</ol>

</blockquote>
</blockquote>
</li>

<li><p>Modify 22.8.7.4 <a href="https://wg21.link/expected.void.assign">[expected.void.assign]</a> as indicated:</p>

<blockquote>
<pre>
constexpr expected&amp; operator=(const expected&amp; rhs);
</pre>
<blockquote>
<p>
-1- <i>Effects</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-3- <i>Remarks</i>: This operator is defined as deleted unless <code>is_copy_assignable_v&lt;E&gt;</code> 
is <code>true</code> and <code>is_copy_constructible_v&lt;E&gt;</code> is <code>true</code>.
<p/>
<ins>-?- This operator is trivial if <code>is_trivially_copy_constructible_v&lt;E&gt;</code>, 
<code>is_trivially_copy_assignable_v&lt;E&gt;</code>, and <code>is_trivially_destructible_v&lt;E&gt;</code> are 
all <code>true</code>.</ins>
</p>
</blockquote>
<pre>
constexpr expected&amp; operator=(expected&amp;&amp; rhs) noexcept(<i>see below</i>);
</pre>
<blockquote>
<p>
-4- <i>Effects</i>: [&hellip;]
<p/>
[&hellip;]
<p/>
-6- <i>Remarks</i>: The exception specification is equivalent to <code>is_nothrow_move_constructible_v&lt;E&gt; &amp;&amp;
is_nothrow_move_assignable_v&lt;E&gt;</code>.
<p/>
-7- This operator is defined as deleted unless <code>is_move_constructible_v&lt;E&gt;</code> is <code>true</code> 
and <code>is_move_assignable_v&lt;E&gt;</code> is <code>true</code>.
<p/>
<ins>-?- This operator is trivial if <code>is_trivially_move_constructible_v&lt;E&gt;</code>, 
<code>is_trivially_move_assignable_v&lt;E&gt;</code>, and <code>is_trivially_destructible_v&lt;E&gt;</code> are 
all <code>true</code>.</ins>
</p>

</blockquote>
</blockquote>
</li>

</ol>







</body>
</html>
