New P/R for 4121

Author: jwakely

xml/issue4121.xml

@@ -50,9 +50,7 @@ for associative containers, because `c.end()` might not be a good hint."
 "It might be suboptimal, but it still works."
 </p>
 
-</discussion>
-
-<resolution>
+<superseded>
 <p>
 This wording is relative to <paper num="N4986"/>.
 </p>
@@ -96,6 +94,87 @@ constexpr auto <i>container-append</i>(Container&amp; c) {     <i>// exposition
 
 </li>
 </ol>
+</superseded>
+
+<note>2025-03-13; Jonathan provides improved wording for Tim's suggestion</note>
+<p>
+It's true that for some cases it might be optimal to use `c.emplace(ref)`
+instead of `c.emplace_hint(c.end(), ref)` 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 `std::set` or `std::map`,
+only for user-defined associative containers that don't have a `from_range_t`
+constructor or a `C(Iter,Sent)` 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 `std::set` 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>
+
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5001"/>.
+</p>
+
+<ol>
+<li><p>Modify <sref ref="[range.utility.conv.general]"/> 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>
+
 </resolution>
 
 </issue>