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<title>Issue 2744: any's in_place constructors</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#C++17">C++17</a> status.</em></p>
<h3 id="2744"><a href="lwg-defects.html#2744">2744</a>. <code>any</code>'s <code>in_place</code> constructors</h3>
<p><b>Section:</b> 22.7.4.2 <a href="https://wg21.link/any.cons">[any.cons]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Ville Voutilainen <b>Opened:</b> 2016-07-10 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#any.cons">issues</a> in [any.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The <code>in_place</code> constructor that takes an <code>initializer_list</code> has both a <i>Requires:</i>
for <code>is_constructible</code> and a <i>Remarks:</i> for <code>is_constructible</code>. The one
that takes just a pack has just a <i>Requires:</i> for <code>is_constructible</code>.
<p/>
I think both of those should be <i>Remarks:</i>, i.e. SFINAEable constraints.
Otherwise querying is_constructible for an any with <code>in_place_t</code> will
not give a reasonable answer, and I utterly fail to see any implementation
burden in SFINAEing those constructors.
</p>
<p><i>[2016-07 Chicago]</i></p>

<p>Monday: P0 - tentatively ready</p>


<p id="res-2744"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4606.</p>

<ol>
<li><p>Modify 22.7.4.2 <a href="https://wg21.link/any.cons">[any.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class ValueType&gt;
  any(ValueType&amp;&amp; value);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-7- <i>Requires:</i> <code>T</code> shall satisfy the <code>CopyConstructible</code> requirements. If 
<code>is_copy_constructible_v&lt;T&gt;</code> is <code>false</code>, the program is ill-formed.
<p/>
-8- <i>Effects:</i> Constructs an object of type <code>any</code> that contains an object of type <code>T</code> 
direct-initialized with <code>std::forward&lt;ValueType&gt;(value)</code>.
<p/>
-9- <i>Remarks:</i> This constructor shall not participate in overload resolution if <code>decay_t&lt;ValueType&gt;</code> is the
same type as <code>any</code> <ins>or if <code>ValueType</code> is a specialization of <code>in_place_type_t</code></ins>.
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;class T, class... Args&gt;
  explicit any(in_place_type_t&lt;T&gt;, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
<del>-11- <i>Requires:</i> <code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code>.</del>
<p/>
<ins>-?- <i>Remarks:</i> This constructor shall not participate in overload resolution unless
<code>is_constructible_v&lt;T, Args...&gt;</code> is <code>true</code></ins>
<p/>
[&hellip;]
</p>
</blockquote>

<pre>
template &lt;class T, class U, class... Args&gt;
  explicit any(in_place_type_t&lt;T&gt;, initializer_list&lt;U&gt; il, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>
<del>-15- <i>Requires:</i> <code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is <code>true</code>.</del>
<p/>
[&hellip;]
<p/>
-19- <i>Remarks:</i> <del>The function</del><ins>This constructor</ins> shall not participate in overload resolution unless 
<code>is_constructible_v&lt;T, initializer_list&lt;U&gt;&amp;, Args...&gt;</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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