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<title>Issue 3684: std::allocator&lt;T&gt;::allocate_at_least in constant evaluation</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="3684"><a href="lwg-active.html#3684">3684</a>. <code>std::allocator&lt;T&gt;::allocate_at_least</code> in constant evaluation</h3>
<p><b>Section:</b> 20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2022-03-22 <b>Last modified:</b> 2022-05-17</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#allocator.members">issues</a> in [allocator.members].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
<code>std::allocator&lt;T&gt;::allocate_at_least</code> is a constexpr function that allocates memory during 
constant evaluation, but its restrictions is not clear. Presumably the restrictions are same as those of 
<code>std::allocator&lt;T&gt;::allocate</code>, and we should specify <code>allocate_at_least</code> in term of <code>allocate</code>.
<p/>
The MSVC STL implementation returns <code>allocation_result&lt;T*&gt;{allocate(n), n}</code> now. Perhaps we 
should adopt this strategy for constant evaluation to avoid additional mechanism in the compiler.
</p>

<p><i>[2022-05-17; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
Suggestion to fix this in Core instead.
</p>



<p id="res-3684"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a>.
</p>

<ol>
<li><p>Modify 20.2.10.2 <a href="https://wg21.link/allocator.members">[allocator.members]</a> as indicated:</p>

<blockquote>
<pre>
[[nodiscard]] constexpr allocation_result&lt;T*&gt; allocate_at_least(size_t n);
</pre>
<blockquote>
<p>
-6- <i>Mandates:</i> <code>T</code> is not an incomplete type (6.9.1 <a href="https://wg21.link/basic.types.general">[basic.types.general]</a>).
<p/>
-7- <i>Returns:</i> <code>allocation_result&lt;T*&gt;{ptr, count}</code>, where <code>ptr</code> is a pointer 
to the initial element of an array of <code>count T</code> and <code>count &ge; n</code>.
<p/>
-8- <i>Throws:</i> <code>bad_array_new_length</code> if <code>numeric_limits&lt;size_t&gt;::max() / sizeof(T) &lt; n</code>, 
or <code>bad_alloc</code> if the storage cannot be obtained.
<p/>
-9- <i>Remarks:</i> The storage for the array is obtained by calling <code>::operator new</code>, 
but it is unspecified when or how often this function is called. This function starts the 
lifetime of the array object, but not that of any of the array elements. <ins>This function returns 
<code>allocation_result&lt;T*&gt;{allocate(n), n}</code> within the evaluation of a core constant expression.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





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