New issue from Hewill Kang: "ranges::to's copy branch is underconstra…

…ined"

xml/issue4018.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4018" status="New">
+<title><tt>ranges::to</tt>'s copy branch is underconstrained</title>
+<section><sref ref="[range.utility.conv.to]"/></section>
+<submitter>Hewill Kang</submitter>
+<date>25 Nov 2023</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+Unlike other branches that return a prvalue <tt>C</tt>, this branch's <tt>C</tt> is returned by elidable move, 
+indicating that <tt>C</tt> needs to be move constructible (<a href="https://godbolt.org/z/dfqnx6Txq">demo</a>):
+</p>
+<blockquote><pre>
+#include &lt;ranges&gt;
+#include &lt;vector&gt;
+
+struct nonmovable {
+  nonmovable() = default;
+  nonmovable(const nonmovable&amp;) = delete;
+  nonmovable&amp; operator=(const nonmovable&amp;) = delete;
+};
+
+template&lt;class T&gt;
+struct nonmovable_vector : std::vector&lt;T&gt;, nonmovable { };
+
+int main() {
+  int arr[] = {42};
+  auto v = std::ranges::to&lt;nonmovable_vector&lt;int&gt;&gt;(arr); // <span style="color:red;font-weight:bolder">hard error</span>
+}
+</pre></blockquote>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N4964"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[range.utility.conv.to]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class C, input_range R, class... Args&gt; requires (!view&lt;C&gt;)
+  constexpr C to(R&amp;&amp; r, Args&amp;&amp;... args);
+</pre>
+<blockquote>
+<p>
+-1- <i>Mandates</i>: <tt>C</tt> is a cv-unqualified class type.
+</p>
+<p>
+-2- <i>Returns</i>: An object of type <tt>C</tt> constructed from the elements of <tt>r</tt> in the following manner:
+</p>
+<ol style="list-style-type: none">
+<li><p>(2.1) &mdash; If <tt>C</tt> does not satisfy <tt>input_range</tt> or 
+<tt>convertible_to&lt;range_reference_t&lt;R&gt;, range_value_t&lt;C&gt;&gt;</tt> is <tt>true</tt>:</p>
+<ol style="list-style-type: none">
+<li><p>(2.1.1) &mdash; [&hellip;]</p></li>
+<li><p>(2.1.2) &mdash; [&hellip;]</p></li>
+<li><p>(2.1.3) &mdash; [&hellip;]</p></li>
+<li><p>(2.1.4) &mdash; Otherwise, if</p>
+<ol style="list-style-type: none">
+<li><p><ins>(2.1.4.?) &mdash; <tt>move_constructible&lt;C&gt;</tt> is <tt>true</tt>,</ins></p></li>
+<li><p>(2.1.4.1) &mdash; <tt>constructible_from&lt;C, Args...&gt;</tt> is <tt>true</tt>, and</p></li>
+<li><p>(2.1.4.2) &mdash; <tt><i>container-insertable</i>&lt;C, range_reference_t&lt;R&gt;&gt;</tt> is <tt>true</tt>:</p>
+<blockquote><pre>
+C c(std::forward&lt;Args&gt;(args)...);
+if constexpr (sized_range&lt;R> &amp;&amp; <i>reservable-container</i>&lt;C&gt;)
+  c.reserve(static_cast&lt;range_size_t&lt;C&gt;&gt;(ranges::size(r)));
+ranges::copy(r, <i>container-inserter</i>&lt;range_reference_t&lt;R&gt;&gt;(c));
+</pre></blockquote></li>
+</ol>
+</li>
+<li><p>(2.1.5) &mdash; Otherwise, the program is ill-formed.</p></li>
+</ol></li>
+<li><p>(2.2) &mdash; [&hellip;]</p></li>
+<li><p>(2.3) &mdash; Otherwise, the program is ill-formed.</p></li>
+</ol>
+
+</blockquote>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>