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<title>Issue 2511: scoped_allocator_adaptor piecewise construction does not require CopyConstructible</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2511"><a href="lwg-defects.html#2511">2511</a>. <code>scoped_allocator_adaptor</code> piecewise construction does not require <code>CopyConstructible</code></h3>
<p><b>Section:</b> 20.6.4 <a href="https://wg21.link/allocator.adaptor.members">[allocator.adaptor.members]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> David Krauss <b>Opened:</b> 2015-06-16 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.adaptor.members">issues</a> in [allocator.adaptor.members].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<p>
20.6.4 <a href="https://wg21.link/allocator.adaptor.members">[allocator.adaptor.members]</a>/10 requires that the argument types in the piecewise-construction tuples 
all be <code>CopyConstructible</code>. These tuples are typically created by <code>std::forward_as_tuple</code>, such as in 
&para;13. So they will be a mix of lvalue and rvalue references, the latter of which are not <code>CopyConstructible</code>.
<p/>
My guess is that <code>CopyConstructible</code> was specified to feed the <code>tuple_cat</code>, before that function could 
handle rvalues. Since the argument tuple is already moved in &para;11, the requirement is obsolete. It should either 
be changed to <code>MoveConstructible</code>, or perhaps better, convert the whole tuple to references (i.e. form 
<code>tuple&lt;Args1&amp;&amp;...&gt;</code>) so nothing needs to be moved. After all, this is a facility for handling non-movable 
types.
<p/>
It appears that the resolution of DR <a href="lwg-defects.html#2203" title="scoped_allocator_adaptor uses wrong argument types for piecewise construction (Status: C++14)">2203</a><sup><a href="https://cplusplus.github.io/LWG/issue2203" title="Latest snapshot">(i)</a></sup>, which added <code>std::move</code> to &para;11, simply omitted the 
change to &para;10.
</p>

<p><i>[2016-11-08, Jonathan comments]</i></p>

<p>
My paper <a href="https://wg21.link/p0475r0">P0475R0</a> provides a proposed resolution.
</p>

<p><i>[2018-06 set to 'Resolved']</i></p>

<p><a href="https://wg21.link/p0475r1">P0475R1</a> was adopted in Rapperswil.</p>


<p id="res-2511"><b>Proposed resolution:</b></p>





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