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<title>Issue 4312: Const and value category mismatch for allocator_arg_t/allocator_arg in the description 
of uses-allocator construction</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#New">New</a> status.</em></p>
<h3 id="4312"><a href="lwg-active.html#4312">4312</a>. Const and value category mismatch for <code class='backtick'>allocator_arg_t</code>/<code class='backtick'>allocator_arg</code> in the description 
of uses-allocator construction</h3>
<p><b>Section:</b> 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-08-06 <b>Last modified:</b> 2025-08-16</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#allocator.uses.construction">active issues</a> in [allocator.uses.construction].</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.uses.construction">issues</a> in [allocator.uses.construction].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> bullet 2.2 states:
</p>
<blockquote><p>
Otherwise, if <code class='backtick'>T</code> has a constructor invocable as <code class='backtick'>T(allocator_arg, alloc, args...)</code> (leading-allocator convention), 
[&hellip;]
</p></blockquote>
<p>
However, when forming construction arguments in the utility functions, we're actually using <i>cv</i>-unqualified 
rvalue of <code class='backtick'>allocator_arg_t</code>, which can be inferred from using plain <code class='backtick'>allocator_arg_t</code> but not 
<code>const allocator_arg_t&amp;</code> in 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> bullet 5.2.
<p/>
It seems that such mismatch was present even since C++11 (per <a href="https://wg21.link/N3337" title=" Working Draft, Standard for Programming Language C++">N3337</a> [allocator.uses.construction]/1.2). 
If the use of plain <code class='backtick'>allocator_arg_t</code> is considered correct, I think we should fix the description.
</p>


<p id="res-4312"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<ol>

<li><p>Modify 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a> as indicated:</p>

<blockquote>
<p>
-2- The following utility functions support three conventions for passing <code class='backtick'>alloc</code> to a constructor:
</p>
<ol style="list-style-type: none">
<li><p>(2.1) &mdash; [&hellip;]</p></li>
<li><p>(2.2) &mdash; Otherwise, if <code class='backtick'>T</code> has a constructor invocable as 
<code>T(<del>allocator_arg</del><ins>allocator_arg_t{}</ins>, alloc, args...)</code> 
(leading-allocator convention), then uses-allocator construction chooses this constructor form.</p></li>
<li><p>(2.3) &mdash; [&hellip;]</p></li>
</ol>
</blockquote>

</li>

</ol>





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