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issue4121.html

@@ -64,7 +64,7 @@
 <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="4121"><a href="lwg-active.html#4121">4121</a>. <code>ranges::to</code> constructs associative containers via <code>c.emplace(c.end(), *it)</code></h3>
 <p><b>Section:</b> 25.5.7.1 <a href="https://wg21.link/range.utility.conv.general">[range.utility.conv.general]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
- <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-16 <b>Last modified:</b> 2024-08-02</p>
+ <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-16 <b>Last modified:</b> 2025-03-17</p>
 <p><b>Priority: </b>2
 </p>
 <p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
@@ -112,9 +112,9 @@ <h3 id="4121"><a href="lwg-active.html#4121">4121</a>. <code>ranges::to</code> c
 "It might be suboptimal, but it still works."
 </p>
 
+<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
+<blockquote class="note">
 
-
-<p id="res-4121"><b>Proposed resolution:</b></p>
 <p>
 This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
 </p>
@@ -158,6 +158,88 @@ <h3 id="4121"><a href="lwg-active.html#4121">4121</a>. <code>ranges::to</code> c
 
 </li>
 </ol>
+</blockquote>
+
+<p><i>[2025-03-13; Jonathan provides improved wording for Tim's suggestion]</i></p>
+
+<p>
+It's true that for some cases it might be optimal to use <code class='backtick'>c.emplace(ref)</code>
+instead of <code class='backtick'>c.emplace_hint(c.end(), ref)</code> but I don't think I care.
+A bad hint is not expensive, it's just an extra comparison then the hint
+is ignored.
+And this code path isn't going to be used for <code class='backtick'>std::set</code> or <code class='backtick'>std::map</code>,
+only for user-defined associative containers that don't have a <code class='backtick'>from_range_t</code>
+constructor or a <code class='backtick'>C(Iter,Sent)</code> constructor.
+I think just fixing the original issue is all we need,
+rather than trying to handle every possible way to insert elements.
+</p>
+<p>
+This is a simpler, portable reproducer that doesn't depend on the current
+implementation status of <code class='backtick'>std::set</code> in libstdc++:
+</p>
+<blockquote><pre><code>
+#include &lt;ranges&gt;
+#include &lt;set&gt;
+struct Set : std::set&lt;int&gt; {
+  Set() { }; // No other constructors
+};
+int main() {
+  int a[1];
+  auto okay = std::ranges::to&lt;std::set&lt;int&gt;&gt;(a);
+  auto ohno = std::ranges::to&lt;Set&gt;(a);
+}
+</code></pre></blockquote>
+
+
+
+<p id="res-4121"><b>Proposed resolution:</b></p>
+<p>
+This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
+</p>
+
+<ol>
+<li><p>Modify 25.5.7.1 <a href="https://wg21.link/range.utility.conv.general">[range.utility.conv.general]</a> as indicated:</p>
+
+<blockquote>
+<p>
+-4- Let <code><i>container-appendable</i></code> be defined as follows:
+</p>
+<blockquote><pre>
+template&lt;class Container, class Ref&gt;
+constexpr bool <i>container-appendable</i> =         <i>// exposition only</i>
+  requires(Container&amp; c, Ref&amp;&amp; ref) {
+           requires (requires { c.emplace_back(std::forward&lt;Ref&gt;(ref)); } ||
+                     requires { c.push_back(std::forward&lt;Ref&gt;(ref)); } ||
+                     <ins>requires { c.emplace_hint(c.end(), std::forward&lt;Ref&gt;(ref)); } ||</ins>
+                     requires { c.emplace(c.end(), std::forward&lt;Ref&gt;(ref)); } ||
+                     requires { c.insert(c.end(), std::forward&lt;Ref&gt;(ref)); });
+};
+</pre></blockquote>
+<p>
+-5- Let <code><i>container-append</i></code> be defined as follows:
+</p>
+<blockquote><pre>
+template&lt;class Container&gt;
+constexpr auto <i>container-append</i>(Container&amp; c) {     <i>// exposition only</i>
+  return [&amp;c]&lt;class Ref&gt;(Ref&amp;&amp; ref) {
+    if constexpr (requires { c.emplace_back(declval&lt;Ref&gt;()); })
+      c.emplace_back(std::forward&lt;Ref&gt;(ref));
+    else if constexpr (requires { c.push_back(declval&lt;Ref&gt;()); })
+      c.push_back(std::forward&lt;Ref&gt;(ref));
+    <ins>else if constexpr (requires { c.emplace_hint(c.end(), declval&lt;Ref&gt;()); })
+      c.emplace_hint(c.end(), std::forward&lt;Ref&gt;(ref));</ins>
+    else if constexpr (requires { c.emplace(c.end(), declval&lt;Ref&gt;()); })
+      c.emplace(c.end(), std::forward&lt;Ref&gt;(ref));
+    else
+      c.insert(c.end(), std::forward&lt;Ref&gt;(ref));
+  };
+};
+</pre></blockquote>
+</blockquote>
+
+</li>
+</ol>
+
 
 
 

lwg-active.html

@@ -67,7 +67,7 @@
 </tr>
 <tr>
   <td align="left">Date:</td>
-  <td align="left">2025-03-16</td>
+  <td align="left">2025-03-17</td>
 </tr>
 <tr>
   <td align="left">Project:</td>
@@ -79,7 +79,7 @@
 </tr>
 </table>
 <h1>C++ Standard Library Active Issues List (Revision D125)</h1>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p>
   <p>Reference ISO/IEC IS 14882:2020(E)</p>
   <p>Also see:</p>
@@ -62330,7 +62330,7 @@ <h3 id="4120"><a href="#4120">4120</a><sup><a href="https://cplusplus.github.io/
 <hr>
 <h3 id="4121"><a href="#4121">4121</a><sup><a href="https://cplusplus.github.io/LWG/issue4121">(i)</a></sup>. <code>ranges::to</code> constructs associative containers via <code>c.emplace(c.end(), *it)</code></h3>
 <p><b>Section:</b> 25.5.7.1 <a href="https://wg21.link/range.utility.conv.general">[range.utility.conv.general]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
- <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-16 <b>Last modified:</b> 2024-08-02</p>
+ <b>Submitter:</b> Hewill Kang <b>Opened:</b> 2024-07-16 <b>Last modified:</b> 2025-03-17</p>
 <p><b>Priority: </b>2
 </p>
 <p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
@@ -62378,9 +62378,9 @@ <h3 id="4121"><a href="#4121">4121</a><sup><a href="https://cplusplus.github.io/
 "It might be suboptimal, but it still works."
 </p>
 
+<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
+<blockquote class="note">
 
-
-<p id="res-4121"><b>Proposed resolution:</b></p>
 <p>
 This wording is relative to <a href="https://wg21.link/N4986" title=" Working Draft, Programming Languages — C++">N4986</a>.
 </p>
@@ -62424,6 +62424,88 @@ <h3 id="4121"><a href="#4121">4121</a><sup><a href="https://cplusplus.github.io/
 
 </li>
 </ol>
+</blockquote>
+
+<p><i>[2025-03-13; Jonathan provides improved wording for Tim's suggestion]</i></p>
+
+<p>
+It's true that for some cases it might be optimal to use <code class='backtick'>c.emplace(ref)</code>
+instead of <code class='backtick'>c.emplace_hint(c.end(), ref)</code> but I don't think I care.
+A bad hint is not expensive, it's just an extra comparison then the hint
+is ignored.
+And this code path isn't going to be used for <code class='backtick'>std::set</code> or <code class='backtick'>std::map</code>,
+only for user-defined associative containers that don't have a <code class='backtick'>from_range_t</code>
+constructor or a <code class='backtick'>C(Iter,Sent)</code> constructor.
+I think just fixing the original issue is all we need,
+rather than trying to handle every possible way to insert elements.
+</p>
+<p>
+This is a simpler, portable reproducer that doesn't depend on the current
+implementation status of <code class='backtick'>std::set</code> in libstdc++:
+</p>
+<blockquote><pre><code>
+#include &lt;ranges&gt;
+#include &lt;set&gt;
+struct Set : std::set&lt;int&gt; {
+  Set() { }; // No other constructors
+};
+int main() {
+  int a[1];
+  auto okay = std::ranges::to&lt;std::set&lt;int&gt;&gt;(a);
+  auto ohno = std::ranges::to&lt;Set&gt;(a);
+}
+</code></pre></blockquote>
+
+
+
+<p id="res-4121"><b>Proposed resolution:</b></p>
+<p>
+This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
+</p>
+
+<ol>
+<li><p>Modify 25.5.7.1 <a href="https://wg21.link/range.utility.conv.general">[range.utility.conv.general]</a> as indicated:</p>
+
+<blockquote>
+<p>
+-4- Let <code><i>container-appendable</i></code> be defined as follows:
+</p>
+<blockquote><pre>
+template&lt;class Container, class Ref&gt;
+constexpr bool <i>container-appendable</i> =         <i>// exposition only</i>
+  requires(Container&amp; c, Ref&amp;&amp; ref) {
+           requires (requires { c.emplace_back(std::forward&lt;Ref&gt;(ref)); } ||
+                     requires { c.push_back(std::forward&lt;Ref&gt;(ref)); } ||
+                     <ins>requires { c.emplace_hint(c.end(), std::forward&lt;Ref&gt;(ref)); } ||</ins>
+                     requires { c.emplace(c.end(), std::forward&lt;Ref&gt;(ref)); } ||
+                     requires { c.insert(c.end(), std::forward&lt;Ref&gt;(ref)); });
+};
+</pre></blockquote>
+<p>
+-5- Let <code><i>container-append</i></code> be defined as follows:
+</p>
+<blockquote><pre>
+template&lt;class Container&gt;
+constexpr auto <i>container-append</i>(Container&amp; c) {     <i>// exposition only</i>
+  return [&amp;c]&lt;class Ref&gt;(Ref&amp;&amp; ref) {
+    if constexpr (requires { c.emplace_back(declval&lt;Ref&gt;()); })
+      c.emplace_back(std::forward&lt;Ref&gt;(ref));
+    else if constexpr (requires { c.push_back(declval&lt;Ref&gt;()); })
+      c.push_back(std::forward&lt;Ref&gt;(ref));
+    <ins>else if constexpr (requires { c.emplace_hint(c.end(), declval&lt;Ref&gt;()); })
+      c.emplace_hint(c.end(), std::forward&lt;Ref&gt;(ref));</ins>
+    else if constexpr (requires { c.emplace(c.end(), declval&lt;Ref&gt;()); })
+      c.emplace(c.end(), std::forward&lt;Ref&gt;(ref));
+    else
+      c.insert(c.end(), std::forward&lt;Ref&gt;(ref));
+  };
+};
+</pre></blockquote>
+</blockquote>
+
+</li>
+</ol>
+
 
 
 

lwg-closed.html

@@ -67,7 +67,7 @@
 </tr>
 <tr>
   <td align="left">Date:</td>
-  <td align="left">2025-03-16</td>
+  <td align="left">2025-03-17</td>
 </tr>
 <tr>
   <td align="left">Project:</td>
@@ -79,7 +79,7 @@
 </tr>
 </table>
 <h1>C++ Standard Library Closed Issues List (Revision D125)</h1>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p>
   <p>Reference ISO/IEC IS 14882:2020(E)</p>
   <p>Also see:</p>

lwg-defects.html

@@ -67,7 +67,7 @@
 </tr>
 <tr>
   <td align="left">Date:</td>
-  <td align="left">2025-03-16</td>
+  <td align="left">2025-03-17</td>
 </tr>
 <tr>
   <td align="left">Project:</td>
@@ -79,7 +79,7 @@
 </tr>
 </table>
 <h1>C++ Standard Library Defect Reports and Accepted Issues (Revision D125)</h1>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p>
   <p>Reference ISO/IEC IS 14882:2020(E)</p>
   <p>Also see:</p>

lwg-immediate.html

@@ -62,7 +62,7 @@ <h1>C++ Standard Library Issues Resolved Directly In [INSERT CURRENT MEETING HER
 </tr>
 <tr>
 <td align="left">Date:</td>
-<td align="left">Revised 2025-03-16 at 09:09:08 UTC
+<td align="left">Revised 2025-03-17 at 15:39:10 UTC
 </td>
 </tr>
 <tr>

lwg-index-open.html

@@ -66,7 +66,7 @@ <h1>Index by Section</h1>
 <p>This document is the Index by Section for the <a href="lwg-active.html">Library Active Issues List</a>.</p>
 <h2>Index by Section (non-Ready active issues only)</h2>
 <p><a href="lwg-index.html">(view all issues)</a></p>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p><h2 id="Section_3">Section 3 (2 issues)</h2>
 <p><a href="lwg-index.html#Section_3">(view all issues)</a></p>
 <table class="issues-index">

lwg-index.html

@@ -66,7 +66,7 @@ <h1>Index by Section</h1>
 <p>This document is the Index by Section for the <a href="lwg-active.html">Library Active Issues List</a>, <a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.</p>
 <h2>Index by Section</h2>
 <p><a href="lwg-index-open.html">(view only non-Ready open issues)</a></p>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p><h2 id="Section_2">Section 2 (2 issues)</h2>
 <table class="issues-index">
 <tr>

lwg-ready.html

@@ -62,7 +62,7 @@ <h1>C++ Standard Library Issues to be moved in [INSERT CURRENT MEETING HERE]</h1
 </tr>
 <tr>
 <td align="left">Date:</td>
-<td align="left">Revised 2025-03-16 at 09:09:08 UTC
+<td align="left">Revised 2025-03-17 at 15:39:10 UTC
 </td>
 </tr>
 <tr>

lwg-status-date.html

@@ -67,7 +67,7 @@ <h1>Index by Status and Date</h1>
 This document is the Index by Status and Date for the <a href="lwg-active.html">Library Active Issues List</a>,
 <a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.
 </p>
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p><h2 id="Ready">Ready (7 issues)</h2>
 <table class="issues-index">
 <tr>
@@ -314,6 +314,15 @@ <h2 id="New">New (464 issues)</h2>
   <th>Duplicates</th>
 </tr>
 <tr>
+<td id="4121"><a href="lwg-active.html#4121" title="ranges::to constructs associative containers via c.emplace(c.end(), *it) (Status: New)">4121</a><sup><a href="https://cplusplus.github.io/LWG/issue4121">(i)</a></sup></td>
+<td><a href="lwg-active.html#New">New</a></td>
+<td>25.5.7.1 [range.utility.conv.general]</td>
+<td><code>ranges::to</code> constructs associative containers via <code>c.emplace(c.end(), *it)</code></td>
+<td>Yes</td>
+<td>2</td>
+<td></td>
+</tr>
+<tr>
 <td id="4226"><a href="lwg-active.html#4226" title="to_input_view::iterator cannot be compared to its const sentinel (Status: New)">4226</a><sup><a href="https://cplusplus.github.io/LWG/issue4226">(i)</a></sup></td>
 <td><a href="lwg-active.html#New">New</a></td>
 <td>99 [range.to.input]</td>
@@ -1040,15 +1049,6 @@ <h2 id="New">New (464 issues)</h2>
 <td></td>
 </tr>
 <tr>
-<td id="4121"><a href="lwg-active.html#4121" title="ranges::to constructs associative containers via c.emplace(c.end(), *it) (Status: New)">4121</a><sup><a href="https://cplusplus.github.io/LWG/issue4121">(i)</a></sup></td>
-<td><a href="lwg-active.html#New">New</a></td>
-<td>25.5.7.1 [range.utility.conv.general]</td>
-<td><code>ranges::to</code> constructs associative containers via <code>c.emplace(c.end(), *it)</code></td>
-<td>Yes</td>
-<td>2</td>
-<td></td>
-</tr>
-<tr>
 <td id="4081"><a href="lwg-active.html#4081" title="concat_view::iterator::operator- is overconstrained (Status: New)">4081</a><sup><a href="https://cplusplus.github.io/LWG/issue4081">(i)</a></sup></td>
 <td><a href="lwg-active.html#New">New</a></td>
 <td>25.7.18.3 [range.concat.iterator]</td>

lwg-status.html

@@ -62,7 +62,7 @@ <h1>Index by Status and Section</h1>
 <a href="lwg-defects.html">Library Defect Reports and Accepted Issues</a>, and <a href="lwg-closed.html">Library Closed Issues List</a>.
 </p>
 
-<p>Revised 2025-03-16 at 09:09:08 UTC
+<p>Revised 2025-03-17 at 15:39:10 UTC
 </p><h2 id="Ready">Ready (7 issues)</h2>
 <table class="issues-index">
 <tr>