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<title>Issue 4267: Uses-allocator construction is meaningless for tuple of references</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="4267"><a href="lwg-active.html#4267">4267</a>. Uses-allocator construction is meaningless for tuple of references</h3>
<p><b>Section:</b> 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-05-24 <b>Last modified:</b> 2025-05-24</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#tuple.cnstr">active issues</a> in [tuple.cnstr].</p>
<p><b>View all other</b> <a href="lwg-index.html#tuple.cnstr">issues</a> in [tuple.cnstr].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Per 20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a>/1, uses-allocator construction is only defined for objects. 
And presumably, an attempt to construct <code class='backtick'>std::tuple</code> of reference from an <code class='backtick'>allocator_arg_t</code> constructor 
causes a hard error.
<p/>
Since C++23/<a href="https://wg21.link/P2255R2" title=" A type trait to detect reference binding to temporary">P2255R2</a>, it seems that these <code class='backtick'>allocator_arg_t</code> constructors are conditionally 
deleted according to 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a>/33. However, it's confusing that these constructors are 
sometimes non-deleted when the tuple contains a reference, while there are hard errors in an instantiation 
instead.
</p>


<p id="res-4267"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>
<ol>

<li><p>Modify 22.4.4.2 <a href="https://wg21.link/tuple.cnstr">[tuple.cnstr]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Alloc&gt;
  constexpr explicit(<i>see below</i>)
    tuple(allocator_arg_t, const Alloc&amp; a);
[&hellip;]
template&lt;class Alloc, tuple-like UTuple&gt;
  constexpr explicit(<i>see below</i>)
    tuple(allocator_arg_t, const Alloc&amp; a, UTuple&amp;&amp;);
</pre>
<blockquote>
<p>
-32- <i>Preconditions</i>: <code class='backtick'>Alloc</code> meets the <i>Cpp17Allocator</i> requirements (16.4.4.6.1 <a href="https://wg21.link/allocator.requirements.general">[allocator.requirements.general]</a>).
<p/>
-33- <i>Effects</i>: Equivalent to the preceding constructors except that each element is constructed with uses-allocator
construction (20.2.8.2 <a href="https://wg21.link/allocator.uses.construction">[allocator.uses.construction]</a>).
<p/>
<ins>-?- <i>Remarks</i>: These constructors are defined as deleted if <code>is_reference_v&lt;T<sub><i>i</i></sub>&gt;</code> 
is <code class='backtick'>true</code> for at least one <code>T<sub><i>i</i></sub></code>.</ins>
</p>
</blockquote>
</blockquote>

</li>

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