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<title>Issue 2855: std::throw_with_nested("string_literal")</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="2855"><a href="lwg-defects.html#2855">2855</a>. <code>std::throw_with_nested("string_literal")</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> Jonathan Wakely <b>Opened:</b> 2017-01-17 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>0
</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>
[except.nested] says:
</p>
<blockquote>
<pre>
template &lt;class T&gt; [[noreturn]] void throw_with_nested(T&amp;&amp; t);
</pre>
<blockquote>
<p>
Let <code>U</code> be <code>remove_reference_t&lt;T&gt;</code>.
<p/>
<i>Requires:</i> <code>U</code> shall be <code>CopyConstructible</code>.
</p>
</blockquote>
</blockquote>
<p>
This forbids <code>std::throw_with_nested("string literal")</code> because <code>T</code> gets deduced as <code>const char(&amp;)[15]</code> 
and so <code>U</code> is <code>const char[15]</code> which is not <code>CopyConstructible</code>.
<p/>
A throw expression decays an array argument to a pointer (7.6.18 <a href="https://wg21.link/expr.throw">[expr.throw]</a> p2) and so works fine with string literals. 
GCC's <code>throw_with_nested</code> also worked fine until I added a <code>static_assert</code> to enforce the 
<code>CopyConstructible</code> requirement.
<p/>
The same problem exists when throwing a function type, which should also decay:
</p>
<blockquote>
<pre>
#include &lt;exception&gt;

void f() { }

int main() {
  std::throw_with_nested(f);
}
</pre>
</blockquote>
<p>
(Note: LWG <a href="lwg-defects.html#1370" title="throw_with_nested should not use perfect forwarding (Status: C++11)">1370</a><sup><a href="https://cplusplus.github.io/LWG/issue1370" title="Latest snapshot">(i)</a></sup> added the <code>remove_reference</code>, which was a step in the right direction but not far enough.)
</p>

<p><i>[2017-01-27 Telecon]</i></p>

<p>Priority 0</p>


<p id="res-2855"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4618">N4618</a>.</p>

<ol>
<li><p>Edit 17.9.8 <a href="https://wg21.link/except.nested">[except.nested]</a> as indicated:</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><del>remove_reference</del><ins>decay</ins>_t&lt;T&gt;</code>.
<p/>
-7- <i>Requires:</i> <code>U</code> shall be <code>CopyConstructible</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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