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<title>Issue 2305: [fund.ts] optional forwarding construction/assignment</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#NAD">NAD</a> status.</em></p>
<h3 id="2305"><a href="lwg-closed.html#2305">2305</a>. [fund.ts] <code>optional</code> forwarding construction/assignment</h3>
<p><b>Section:</b> 5.3.1 <a href="https://cplusplus.github.io/fundamentals-ts/v1.html#optional.object.ctor">[fund.ts::optional.object.ctor]</a> <b>Status:</b> <a href="lwg-active.html#NAD">NAD</a>
 <b>Submitter:</b> Cassio Neri <b>Opened:</b> 2013-09-23 <b>Last modified:</b> 2015-10-26</p>
<p><b>Priority: </b>4
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD">NAD</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts</b></p>

<p>
Consider:
</p>
<blockquote><pre>
struct foo {
  foo(std::initializer_list&lt;int&gt;&amp;);        // 1
  foo(const std::initializer_list&lt;int&gt;&amp;);  // 2
  foo(std::initializer_list&lt;int&gt;&amp;&amp;);       // 3
  foo(const std::initializer_list&lt;int&gt;&amp;&amp;); // 4
};

std::initializer_list&lt;int&gt; il{0, 1, 2};

foo foo_0{1, 2, 3};                                 // calls 3
foo foo_1{il};                                      // calls 1
foo foo_2((const std::initializer_list&lt;int&gt;&amp;) il);  // calls 2
foo foo_3{(std::initializer_list&lt;int&gt;&amp;&amp;) il};       // calls 3
foo foo_4((const std::initializer_list&lt;int&gt;&amp;&amp;) il); // calls 4
</pre></blockquote>
<p>
Although the constructors of <code>foo</code> are unusual (<code>initializer_list</code>s are <i>normally</i> passed by
value) users of <code>optional</code> could naturally expect perfect forwarding of <code>initializer_list</code>s. However,
all lines below end up calling 1.
</p>
<blockquote><pre>
optional&lt;foo&gt; opt0{in_place, {1, 2, 3}};
optional&lt;foo&gt; opt1{in_place, il};                    
optional&lt;foo&gt; opt3{in_place, (const std::initializer_list&lt;int&gt;&amp;) il};
optional&lt;foo&gt; opt2{in_place, (std::initializer_list&lt;int&gt;&amp;&amp;) il};
optional&lt;foo&gt; opt4{in_place, (const std::initializer_list&lt;int&gt;&amp;&amp;) il};

opt0.emplace({1, 2, 3});
opt0.emplace(il);
opt0.emplace((const std::initializer_list&lt;int&gt;&amp;) il);
opt0.emplace((std::initializer_list&lt;int&gt;&amp;&amp;) il);
opt0.emplace((const std::initializer_list&lt;int&gt;&amp;&amp;) il);
</pre></blockquote>
<p>
The constructor
</p>
<blockquote><pre>
template &lt;class... Args&gt; constexpr explicit optional(in_place_t, Args&amp;&amp;... args);
</pre></blockquote>
<p>
can handle all constructor calls above, except the one taking <code>{1, 2, 3}</code>. Hence, a simple
<ins>modification</ins> of
</p>
<blockquote><pre>
template &lt;class U, class... Args&gt;
constexpr explicit optional(in_place_t, initializer_list&lt;U&gt;<ins>&amp;&amp;</ins> il, Args&amp;&amp;... args);
</pre></blockquote>
<p>
allows perfect forwarding of <code>std::initializer_list&lt;U&gt;</code>s to be complete.
</p>

<p><i>[2014-06-06 pre-Rapperswil]</i></p>

<p>
This issue has been reopened as fundamentals-ts.
</p>

<p><i>[2014-06-17, Rapperswil]</i></p>

<p>
Move to NAD
</p>


<p id="res-2305"><b>Proposed resolution:</b></p>
<p>This wording is relative to N3691.</p>

<ol>
<li><p>Change 5.3.1 <a href="https://cplusplus.github.io/fundamentals-ts/v1.html#optional.object.ctor">[fund.ts::optional.object.ctor]</a> as indicated:</p>

<blockquote><pre>
template &lt;class U, class... Args&gt; 
constexpr explicit optional(in_place_t, initializer_list&lt;U&gt;<ins>&amp;&amp;</ins> il, Args&amp;&amp;... args);
</pre><blockquote>
<p>
-27- <i>Requires</i>: <code>is_constructible&lt;T, initializer_list&lt;U&gt;&amp;<ins>&amp;</ins>, Args&amp;&amp;...>::value</code> is <code>true.</code>
<p/>
-28- <i>Effects</i>: Initializes the contained value as if constructing an object of type <code>T</code> with the arguments
<code><del>il</del><ins>std::move(il)</ins></code>, <code>std::forward&lt;Args&gt;(args)...</code>.
<p/>
[&hellip;]
<p/>
<del>-31- <i>Remarks</i>: The function shall not participate in overload resolution unless
<code>is_constructible&lt;T, initializer_list&lt;U&gt;&amp;, Args&amp;&amp;...&gt;::value</code> is <code>true</code>.</del>
</p>
</blockquote>
</blockquote>
</li>
<li><p>Change 5.3.3 <a href="https://cplusplus.github.io/fundamentals-ts/v1.html#optional.object.assign">[fund.ts::optional.object.assign]</a> as indicated:</p>

<blockquote><pre>
template &lt;class U, class... Args&gt;
void optional&lt;T&gt;::emplace(initializer_list&lt;U&gt;<ins>&amp;&amp;</ins> il, Args&amp;&amp;... args);
</pre><blockquote>
<p>
-27- <i>Requires</i>: <code>is_constructible&lt;T, initializer_list&lt;U&gt;&amp;<ins>&amp;</ins>, Args&amp;&amp;...>::value</code> is <code>true.</code>
<p/>
-28- <i>Effects</i>: Calls <code>*this = nullopt</code>. Then initializes the contained value as if constructing an object of
type <code>T</code> with the arguments <code><del>il</del><ins>std::move(il)</ins></code>, <code>std::forward&lt;Args&gt;(args)...</code>.
<p/>
[&hellip;]
<p/>
<del>-32- <i>Remarks</i>: This function shall not participate in overload resolution unless
<code>is_constructible&lt;T, initializer_list&lt;U&gt;&amp;, Args&amp;&amp;...>::value</code> is <code>true.</code></del>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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