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<title>Issue 4195: expected&lt;int, int&gt; isn't specified to be trivially assignable</title>
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<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="4195"><a href="lwg-active.html#4195">4195</a>. <code>expected&lt;int, int&gt;</code> isn't specified to be trivially assignable</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> Barry Revzin <b>Opened:</b> 2025-01-21 <b>Last modified:</b> 2025-02-07</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, we specify that the copy constructor, move constructor, and destructor <code>expected&lt;int, int&gt;</code> 
are trivial operations. But we do not specify that the copy assignment operator or move assignment operator are. 
There is implementation divergence &mdash; MSVC's implementation is trivially copyable, but libstdc++'s and libc++'s 
are not (although, they are trivial for the purposes of calls, which is important for being able to return such a 
type in a register).
<p/>
I'm not sure there is any reason the assignment operators should not be trivial here. We should add the same kind 
of remarks in both cases: that the copy assignment operator is trivial if <code>T</code> and <code>E</code> are trivially 
copy assignable and the move assignment operator is trivial if <code>T</code> and <code>E</code> are trivially move assignable.
</p>

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

<p>
Set priority to 2 after reflector poll.
</p>
<p>
"Needs to consider trivial constructors as well as assignment."
</p>
<p>
"This is an ABI break for something that shipped in C++23, NAD."
</p>



<p id="res-4195"><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 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/>
-3- <i>Returns</i>: <code>*this</code>.
<p/>
-4- <i>Remarks</i>: This operator is defined as deleted unless: 
</p>
<ol style="list-style-type: none">
<li><p>(4.1) &mdash; [&hellip;]</p></li>
<li><p>(4.2) &mdash; [&hellip;]</p></li>
<li><p>(4.3) &mdash; [&hellip;]</p></li>
<li><p>(4.4) &mdash; [&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_assignable_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;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/>
-6- <i>Effects</i>: [&hellip;]
<p/>
-7- <i>Returns</i>: <code>*this</code>.
<p/>
-8- <i>Remarks</i>: The exception specification is equivalent to:
</p>
<blockquote><pre>
is_nothrow_move_assignable_v&lt;T&gt; &amp;&amp; is_nothrow_move_constructible_v&lt;T&gt; &amp;&amp;
is_nothrow_move_assignable_v&lt;E&gt; &amp;&amp; is_nothrow_move_constructible_v&lt;E&gt;
</pre></blockquote>
<p>
<ins>-?- This operator is trivial if</ins>
</p>
<ol style="list-style-type: none">
<li><p><ins>(?.1) &mdash; <code>is_trivially_move_assignable_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;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/>
-2- <i>Returns</i>: <code>*this</code>.
<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_assignable_v&lt;E&gt;</code> is <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>Constraints</i>: [&hellip;]
<p/>
-5- <i>Effects</i>: [&hellip;]
<p/>
-6- <i>Returns</i>: <code>*this</code>.
<p/>
-7- <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/>
<ins>-?- This operator is trivial if <code>is_trivially_move_assignable_v&lt;E&gt;</code> is <code>true</code>.</ins>
</p>
</blockquote>
</blockquote>
</li>

</ol>





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