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<title>Issue 2483: throw_with_nested() should use is_final</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="2483"><a href="lwg-defects.html#2483">2483</a>. <code>throw_with_nested()</code> should use <code>is_final</code></h3>
<p><b>Section:</b> 17.9.8 <a href="https://wg21.link/except.nested">[except.nested]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Stephan T. Lavavej <b>Opened:</b> 2015-03-27 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#except.nested">issues</a> in [except.nested].</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>
When <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2559.htm">N2559</a> was voted into the Working Paper, 
it said "This function template must take special case to handle non-class types, unions and <code>[[final]]</code> classes that cannot 
be derived from, and [...]".  However, its Standardese didn't handle <code>final</code> classes, and this was never revisited. Now that 
we have <code>is_final</code>, we can achieve this proposal's original intention.
<p/>
Additionally, we need to handle the case where <code>U</code> is <code>nested_exception</code> itself. <code>is_base_of</code>'s wording 
handles this and ignores <i>cv</i>-qualifiers. (Note that <code>is_class</code> detects "non-union class type".)
</p>

<p><i>[2015-05, Lenexa]</i></p>

<p>
STL, MC and JW already do this<br/>
MC: move to Ready, bring to motion on Friday<br/>
7 in favor, none opposed 
</p>


<p id="res-2483"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4296.</p>

<ol>
<li><p>Change 17.9.8 <a href="https://wg21.link/except.nested">[except.nested]</a> as depicted:</p>

<blockquote>
<pre>
template &lt;class T&gt; [[noreturn]] void throw_with_nested(T&amp;&amp; t);
</pre>
<blockquote>
<p>
-6- Let <code>U</code> be <code>remove_reference_t&lt;T&gt;</code>.
<p/>
-7- Requires: <code>U</code> shall be <code>CopyConstructible</code>.
<p/>
-8- Throws: if <del><code>U</code> is a non-union class type not derived from 
<code>nested_exception</code></del><ins><code>is_class&lt;U&gt;::value &amp;&amp; !is_final&lt;U&gt;::value &amp;&amp; 
!is_base_of&lt;nested_exception, U&gt;::value</code> is <code>true</code></ins>, an exception of unspecified type that 
is publicly derived from both <code>U</code> and <code>nested_exception</code> and constructed from 
<code>std::forward&lt;T&gt;(t)</code>, otherwise <code>std::forward&lt;T&gt;(t)</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>






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