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<title>Issue 2745: [fund.ts.v2] Implementability of LWG 2451</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#TS">TS</a> status.</em></p>
<h3 id="2745"><a href="lwg-defects.html#2745">2745</a>. [fund.ts.v2] Implementability of LWG 2451</h3>
<p><b>Section:</b> 5.3 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#optional.object">[fund.ts.v2::optional.object]</a> <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2016-07-10 <b>Last modified:</b> 2018-07-08</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#fund.ts.v2::optional.object">issues</a> in [fund.ts.v2::optional.object].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TS">TS</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts.v2</b></p>
<p>
LWG <a href="lwg-defects.html#2451" title="[fund.ts.v2] optional&lt;T&gt; should 'forward' T's implicit conversions (Status: TS)">2451</a><sup><a href="https://cplusplus.github.io/LWG/issue2451" title="Latest snapshot">(i)</a></sup> adds conditionally explicit converting constructors to <code>optional&lt;T&gt;</code> that accept:
</p>
<ol>
<li>Types convertible to <code>T</code>: <code>template &lt;class U&gt; constexpr optional(T&amp;&amp;);</code></li>
<li>Rvalue <code>optional&lt;U&gt;</code> when <code>U&amp;&amp;</code> is convertible to <code>T</code>: 
<code>template &lt;class U&gt; constexpr optional(optional&lt;U&gt;&amp;&amp;);</code></li>
<li>Lvalue <code>const optional&lt;U&gt;</code> when <code>const U&amp;</code> is convertible to <code>T</code>: 
<code>template &lt;class U&gt; constexpr optional(const optional&lt;U&gt;&amp;);</code></li>
</ol>
<p>
All three of these constructors are required to be <code>constexpr</code> "If <code>T</code>'s selected constructor is a 
<code>constexpr</code> constructor". While this is not problematic for #1, it is not possible in the current language 
to implement signatures #2 and #3 as <code>constexpr</code> functions for the same reasons that <code>optional</code>'s 
<em>non-converting</em> constructors from <code>optional&lt;T&gt;&amp;&amp;</code> and <code>const optional&lt;T&gt;&amp;</code> 
cannot be <code>constexpr</code>.
<p/>
We should remove the "<code>constexpr</code>" specifier from the declarations of the conditionally explicit converting 
constructors that accept <code>optional&lt;U&gt;&amp;&amp;</code> and <code>const optional&lt;U&gt;&amp;</code>, and strike 
the remarks requiring these constructors to be <code>constexpr</code>.
</p>
<p><i>[2016-07 Chicago]</i></p>

<p>Monday: P0 - tentatively ready</p>
<p>This needs to be considered for C++17 as well</p>


<p id="res-2745"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4600.html">N4600</a>.</p>

<blockquote class="note">
<p>
Wording relative to N4600 + LWG <a href="lwg-defects.html#2451" title="[fund.ts.v2] optional&lt;T&gt; should 'forward' T's implicit conversions (Status: TS)">2451</a><sup><a href="https://cplusplus.github.io/LWG/issue2451" title="Latest snapshot">(i)</a></sup>, although it should be noted that this resolution should be applied 
wherever LWG <a href="lwg-defects.html#2451" title="[fund.ts.v2] optional&lt;T&gt; should 'forward' T's implicit conversions (Status: TS)">2451</a><sup><a href="https://cplusplus.github.io/LWG/issue2451" title="Latest snapshot">(i)</a></sup> is applied, be that to the fundamentals TS or the specification of <code>optional</code> in the 
C++ Working Paper.
</p>
</blockquote>

<ol>
<li><p>Edit 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> as indicated:</p>

<blockquote>
<pre>
template &lt;class T&gt;
class optional
{
public:
  typedef T value_type;

  // 5.3.1, Constructors
  [&hellip;]
  template &lt;class U&gt; constexpr optional(U&amp;&amp;);
  template &lt;class U&gt; <del>constexpr</del> optional(const optional&lt;U&gt;&amp;);
  template &lt;class U&gt; <del>constexpr</del> optional(optional&lt;U&lt;&amp;&amp;);
  [&hellip;]
};
</pre>
</blockquote>
</li>

<li><p>In 5.3.1 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#optional.object.ctor">[fund.ts.v2::optional.object.ctor]</a>, modify the new signature specifications added by LWG <a href="lwg-defects.html#2451" title="[fund.ts.v2] optional&lt;T&gt; should 'forward' T's implicit conversions (Status: TS)">2451</a><sup><a href="https://cplusplus.github.io/LWG/issue2451" title="Latest snapshot">(i)</a></sup></p>

<blockquote>
<pre>
template &lt;class U&gt;
  <del>constexpr</del> optional(const optional&lt;U&gt;&amp; rhs);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-48- <i>Remarks:</i> <del>If <code>T</code>'s selected constructor is a <code>constexpr</code> constructor, this constructor 
shall be a <code>constexpr</code> constructor.</del> This constructor shall not participate in overload resolution 
unless [&hellip;]
</p>
</blockquote>
<pre>
template &lt;class U&gt;
  <del>constexpr</del> optional(optional&lt;U&gt;&amp;&amp; rhs);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-53- <i>Remarks:</i> <del>If <code>T</code>'s selected constructor is a <code>constexpr</code> constructor, this constructor 
shall be a <code>constexpr</code> constructor.</del> This constructor shall not participate in overload resolution 
unless [&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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