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<title>Issue 3888: Most ranges uninitialized memory algorithms are underconstrained</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="3888"><a href="lwg-active.html#3888">3888</a>. Most ranges uninitialized memory algorithms are underconstrained</h3>
<p><b>Section:</b> 26.11.8 <a href="https://wg21.link/specialized.construct">[specialized.construct]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2023-02-17 <b>Last modified:</b> 2023-03-22</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#specialized.construct">issues</a> in [specialized.construct].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
LWG <a href="lwg-defects.html#3870" title="Remove voidify (Status: C++23)">3870</a><sup><a href="https://cplusplus.github.io/LWG/issue3870" title="Latest snapshot">(i)</a></sup> removed the support for constructing objects via <i>cv</i>-qualified lvalues. 
However, it had not changed anything in the immediate context, which means some previously permitted 
usages become hard errors instead of substitution failures.
<p/>
Note that <code>ranges::uninitialized_default_construct</code> and <code>ranges::uninitialized_default_construct_n</code> 
are underconstrained even before LWG <a href="lwg-defects.html#3870" title="Remove voidify (Status: C++23)">3870</a><sup><a href="https://cplusplus.github.io/LWG/issue3870" title="Latest snapshot">(i)</a></sup>, because <code>remove_reference_t&lt;iter_reference_t&lt;I&gt;&gt;</code> 
may be a <code>const</code> type, while only it's <i>cv</i>-unqualified version (<code>iter_value_t&lt;I&gt;</code>) 
is required to be <code>default_initializable</code>.
<p/>
<code>construct_at</code> and <code>ranges::construct_at</code> are also made underconstrained because the function 
body is no longer valid when <code>T</code> is <i>cv</i>-qualified, which is not reflected in <i>Constraints:</i>.
</p>

<p><i>[2023-03-22; Reflector poll]</i></p>

<p>
Set priority to 3 after reflector poll.
</p>



<p id="res-3888"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 26.11.2 <a href="https://wg21.link/special.mem.concepts">[special.mem.concepts]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class I&gt;
concept <i>nothrow-input-iterator</i> = // <i>exposition only</i>
  input_iterator&lt;I&gt; &amp;&amp;
  is_lvalue_reference_v&lt;iter_reference_t&lt;I&gt;&gt; &amp;&amp;
  <ins>same_as&lt;remove_cvref_t&lt;iter_reference_t&lt;I&gt;&gt;, remove_reference_t&lt;iter_reference_t&lt;I&gt;&gt; &amp;&amp;</ins>
  same_as&lt;remove_cvref_t&lt;iter_reference_t&lt;I&gt;&gt;, iter_value_t&lt;I&gt;&gt;;
</pre>
</blockquote>

</li>

<li><p>Modify 26.11.8 <a href="https://wg21.link/specialized.construct">[specialized.construct]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class T, class... Args&gt;
  constexpr T* construct_at(T* location, Args&amp;&amp;... args);

namespace ranges {
  template&lt;class T, class... Args&gt;
    constexpr T* construct_at(T* location, Args&amp;&amp;... args);
}
</pre>
<blockquote>
<p>
-1- <i>Constraints</i>: <ins><code>T</code> is a <i>cv</i>-unqualified type, and t</ins><del>T</del>he expression 
<code>::new (declval&lt;void*&gt;()) T(declval&lt;Args&gt;()...)</code> is well-formed when treated 
as an unevaluated operand (7.2.3 <a href="https://wg21.link/expr.context">[expr.context]</a>).
<p/>
-2- <i>Effects:</i> Equivalent to:
</p>
<blockquote><pre>
return ::new (<i>voidify</i>(*location)) T(std::forward&lt;Args&gt;(args)...);
</pre></blockquote>
</blockquote>
</blockquote>

</li>

</ol>





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