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<title>Issue 2746: Inconsistency between requirements for emplace between optional and variant</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="2746"><a href="lwg-active.html#2746">2746</a>. Inconsistency between requirements for <code>emplace</code> between <code>optional</code> and <code>variant</code></h3>
<p><b>Section:</b> 22.5.3.4 <a href="https://wg21.link/optional.assign">[optional.assign]</a>, 22.6.3.5 <a href="https://wg21.link/variant.mod">[variant.mod]</a>, 22.7.4.4 <a href="https://wg21.link/any.modifiers">[any.modifiers]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2016-07-13 <b>Last modified:</b> 2020-05-10</p>
<p><b>Priority: </b>3
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Referring to N4604:
</p>
<p>
In  [optional.object.assign]: <code>emplace</code> (normal form) has a Requires that the construction works.
</p>
<blockquote>
<p>
<i>Requires:</i> <code>is_constructible_v&lt;T, Args&amp;&amp;...&gt;</code> is <code>true</code>.
</p>
</blockquote>
<p>
<code>emplace</code> (<code>initializer_list</code> form) has a SFINAE condition:
</p>
<blockquote>
<p>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args&amp;&amp;...&gt;</code> 
is <code>true</code>.
</p>
</blockquote>
<p>
In 22.7.4.4 <a href="https://wg21.link/any.modifiers">[any.modifiers]</a>: <code>emplace</code> (normal form) has a Requires that the construction works:
</p>
<blockquote>
<p>
<i>Requires:</i> <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>.
</p>
</blockquote>
<p>
<code>emplace</code> (<code>initializer_list</code> form) has a SFINAE condition:
</p>
<blockquote>
<p>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is <code>true</code>.
</p>
</blockquote>
<p>
In 22.6.3.5 <a href="https://wg21.link/variant.mod">[variant.mod]</a>: <code>emplace</code> (<code>T</code>, normal form) has a SFINAE condition:
</p>
<blockquote>
<p>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>, and <code>T</code> occurs exactly 
once in <code>Types...</code>.
</p>
</blockquote>
<p>
<code>emplace</code> (Idx, normal form) has a <em>both</em> a Requires and a SFINAE condition:
</p>
<blockquote>
<p>
<i>Requires:</i> <code>I &lt; sizeof...(Types)</code>
<p/>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>, and <code>T</code> 
occurs exactly once in <code>Types...</code>.
</p>
</blockquote>
<p>
<code>emplace</code> (<code>T</code>, <code>initializer_list</code> form) has a SFINAE condition:
</p>
<blockquote>
<p>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is 
<code>true</code>, and <code>T</code> occurs exactly once in <code>Types...</code>.
</p>
</blockquote>
<p>
<code>emplace</code> (Idx, <code>initializer_list</code> form) has a both a Requires and a SFINAE condition:
</p>
<blockquote>
<p>
<i>Requires:</i> <code>I &lt; sizeof...(Types)</code>
<p/>
<i>Remarks:</i> [&hellip;] unless <code>is_constructible_v&lt;T, Args...&gt;</code> is true, and <code>T</code> occurs exactly 
once in <code>Types...</code>.
</p>
</blockquote>
<p>
Why the inconsistency? Should all the cases have a SFINAE requirement?
<p/>
I see that variant has an additional requirement (<code>T</code> occurs exactly once in <code>Types...</code>), but that 
only agues that it must be a SFINAE condition &mdash; doesn't say that the other cases (any/variant) should not.
<p/>
<code>map</code>/<code>multimap</code>/<code>unordered_map</code>/<code>unordered_multimap</code> have SFINAE'd versions of 
<code>emplace</code> that don't take <code>initializer_list</code>s, but they don't have any <code>emplace</code> versions 
that take ILs.
<p/>
Suggested resolution:
<p/>
Add SFINAE requirements to <code>optional::emplace(Args&amp;&amp;... args)</code> and 
<code>any::emplace(Args&amp;&amp;... args);</code>
</p>

<p><i>[2016-08 Chicago]</i></p>

<p>During issue prioritization, people suggested that this might apply to <code>any</code> as well.</p>
<p>Ville notes that <a href="lwg-active.html#2746" title="Inconsistency between requirements for emplace between optional and variant (Status: New)">2746</a><sup><a href="https://cplusplus.github.io/LWG/issue2746" title="Latest snapshot">(i)</a></sup>, <a href="lwg-defects.html#2754" title="The in_place constructors and emplace functions added by P0032R3 don't require CopyConstructible (Status: Resolved)">2754</a><sup><a href="https://cplusplus.github.io/LWG/issue2754" title="Latest snapshot">(i)</a></sup> and <a href="lwg-defects.html#2756" title="C++ WP optional&lt;T&gt; should 'forward' T's implicit conversions (Status: C++17)">2756</a><sup><a href="https://cplusplus.github.io/LWG/issue2756" title="Latest snapshot">(i)</a></sup> all go together.</p>

<p><i>[2020-05-10; Daniel comments and provides wording]</i></p>

<p>
The inconsistency between the two <code>any::emplace</code> overloads have been removed by resolving LWG
<a href="lwg-defects.html#2754" title="The in_place constructors and emplace functions added by P0032R3 don't require CopyConstructible (Status: Resolved)">2754</a><sup><a href="https://cplusplus.github.io/LWG/issue2754" title="Latest snapshot">(i)</a></sup> to use <i>Constraints:</i> elements. The last Mandating paper 
(<a href="https://wg21.link/p1460r1">P1460R1</a>), adopted in Prague, changed the <i>Requires:</i> elements
for <code>variant::emplace</code>, "<code>I &lt; sizeof...(Types)</code>" to <i>Mandates:</i>, but that paper
was focused on fixing inappropriate preconditions, not searching for consistency here. Given that the
<code>in_place_index_t</code> constructors of <code>variant</code> uses SFINAE-conditions for this form of static
precondition violation, I recommend that its <code>emplace</code> functions use the same style, which would bring 
them also in consistency with their corresponding type-based <code>emplace</code> forms that are 
<i>Mandates:</i>-free but delegate to the index-based forms.
</p>


<p id="res-2746"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.</p>

<ol>
<li><p>Modify 22.5.3.4 <a href="https://wg21.link/optional.assign">[optional.assign]</a>, as indicated:</p>

<blockquote>
<pre>
template&lt;class... Args&gt; T&amp; emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-29- <i><del>Mandates</del><ins>Constraints</ins>:</i> <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class U, class... Args&gt; T&amp; emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-35- <i>Constraints:</i> <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 22.6.3.5 <a href="https://wg21.link/variant.mod">[variant.mod]</a>, as indicated:</p>

<blockquote>
<pre>
template&lt;class T, class... Args&gt; T&amp; emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-1- <i>Constraints:</i> <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>, and <code>T</code> 
occurs exactly once in <code>Types</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class T, class U, class... Args&gt; T&amp; emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
-3- <i>Constraints:</i> <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is <code>true</code>, 
and <code>T</code> occurs exactly once in <code>Types</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;size_t I, class... Args&gt;
  variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp; emplace(Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
<del>-5- <i>Mandates:</i> <code>I &lt; sizeof...(Types)</code>.</del>
<p/>
-6- <i>Constraints:</i> <code>is_constructible_v&lt;T<sub><i>I</i></sub>, Args...&gt;</code> is <code>true</code> 
<ins>and <code>I &lt; sizeof...(Types)</code> is <code>true</code></ins>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;size_t I, class U, class... Args&gt;
  variant_alternative_t&lt;I, variant&lt;Types...&gt;&gt;&amp; emplace(initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
<del>-12- <i>Mandates:</i> <code>I &lt; sizeof...(Types)</code>.</del>
<p/>
-13- <i>Constraints:</i> <code>is_constructible_v&lt;T<sub><i>I</i></sub>, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is 
<code>true</code> <ins>and <code>I &lt; sizeof...(Types)</code> is <code>true</code></ins>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

</ol>




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