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<title>Issue 3613: Specify that nullopt_t is copyable</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="3613"><a href="lwg-active.html#3613">3613</a>. Specify that <code>nullopt_t</code> is copyable</h3>
<p><b>Section:</b> 22.5.5 <a href="https://wg21.link/optional.nullopt">[optional.nullopt]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Frank Birbacher <b>Opened:</b> 2021-10-01 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#optional.nullopt">issues</a> in [optional.nullopt].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The Standard defines a number of types that are used to create overload disambiguators for constructors, 
like <code>nullopt_t</code> and <code>allocator_tag_t</code>. Values of such types are passed by value to such 
constructors to give it particular meaning. For pass-by-value these types need to be copy-constructible 
and for consistency should also be copy-assignable. Of those types the specification of <code>nullopt_t</code> 
doesn't clearly state that the type is copyable, 22.5.5 <a href="https://wg21.link/optional.nullopt">[optional.nullopt]</a>.
<p/>
The reason that <code>nullopt_t</code> is defined differently from other such types is to avoid ambiguity 
in expressions that assign an empty brace initializer to an <code>optional</code>.

<span style="text-decoration: line-through">
The meaning of such 
assignment should be to engage the <code>optional</code> instead of taking the braces to create a temporary 
<code>nullopt_t</code> for assignment and thus reset the <code>optional</code>.
</span>
The RHS of such assignment should be a temporary empty <code>optional</code>
instead of a temporary <code>nullopt_t</code>.
<p/>
Types that aren't affected: <code>nullptr_t</code> (fundamental type), <code>allocator_tag_t</code>, 
<code>piecewise_construct_t</code>, <code>in_place_t</code>, <code>in_place_type_t</code>, <code>in_place_index_t</code> 
(all basically defined as a <code>class <i>T</i> { explicit <i>T</i>() = default; }</code> which works fine for pass-by-value)
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>
<p><i>[Daniel commented:]</i></p>

<p>
I would prefer to see the wording use
<code>is_trivially_copy_constructible_v</code>
and <code>trivially_copy_assignable_v</code>,
which is consistent with similar usage of trivial-requirements in
<code>std::optional</code>.
</p>
<p><i>[Tim commented:]</i></p>

<p>
We need to say that it models copyable and is trivially copyable
(not sure if we need the latter but might as well - does anyone do it differently?).
"has a copy constructor" isn't enough - <code>T(T&amp;)</code> is a copy constructor.
</p>



<p id="res-3613"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 22.5.5 <a href="https://wg21.link/optional.nullopt">[optional.nullopt]</a> as indicated:</p>

<blockquote>
<pre>
struct nullopt_t{<i>see below</i>};
inline constexpr nullopt_t nullopt(<i>unspecified</i>);
</pre>
<blockquote>
<p>
-1- The struct <code>nullopt_t</code> is an empty class type used as a unique type to indicate 
the state of not containing a value for <code>optional</code> objects. In particular, 
<code>optional&lt;T&gt;</code> has a constructor with <code>nullopt_t</code> as a single
argument; this indicates that an optional object not containing a value shall be constructed.
<p/>
-2- Type <code>nullopt_t</code> shall not have a default constructor or an initializer-list 
constructor, <del>and</del> shall not be an aggregate<ins>, and shall have a copy constructor 
and a copy assignment operator, both shall be public and trivial</ins>.
</p>
</blockquote>
</blockquote>
</li>
</ol>






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