New issue from Hui Xie: "flat_map's transparent comparator no longer works for string literals"

Author: Dani-Hub

xml/issue4239.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4239" status="New">
+<title>`flat_map`'s transparent comparator no longer works for string literals</title>
+<section>
+<sref ref="[flat.map.modifiers]"/>
+</section>
+<submitter>Hui Xie</submitter>
+<date>29 Mar 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+According to the spec, the following code should hard error
+</p>
+<blockquote><pre>
+std::flat_map&lt;std::string, int, std::less&lt;&gt;&gt; m;
+m.try_emplace("abc", 5);  // <span style="color:red;font-weight:bolder">hard error</span>
+</pre></blockquote>
+<p>
+The reason is that we specify in <sref ref="[flat.map.modifiers]"/> p21 the effect to be 
+as if `ranges::upper_bound` is called.
+</p>
+
+<p>
+`ranges::upper_bound` requires `indirect_strict_weak_order`, which requires the comparator to be 
+invocable for all combinations. In this case, it requires
+</p>
+<blockquote><pre>
+const char (&amp;)[4] &lt; const char (&amp;)[4] 
+</pre></blockquote>
+<p>
+to be well-formed, which is no longer the case in C++26 after <paper num="P2865R6"/>.
+<p/>
+We should just use `std::upper_bound` instead. libstdc++ already uses `std::upper_bound`. 
+libc++ uses `ranges::upper_bound` but clang has not yet implemented <paper num="P2865"/> 
+properly.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5008"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[flat.map.modifiers]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+template&lt;class K, class... Args&gt;
+  constexpr pair&lt;iterator, bool&gt; try_emplace(K&amp;&amp; k, Args&amp;&amp;... args);
+template&lt;class K, class... Args&gt;
+  constexpr iterator try_emplace(const_iterator hint, K&amp;&amp; k, Args&amp;&amp;... args);
+</pre>
+<blockquote>
+<p>
+-19- <i>Constraints</i>: [&hellip;]
+<p/>
+-20- <i>Preconditions</i>: [&hellip;]
+<p/>
+-21- <i>Effects</i>: If the map already contains an element whose key is equivalent to `k`, 
+`*this` and `args...` are unchanged. Otherwise equivalent to:
+</p>
+<blockquote><pre>
+auto key_it = <ins>upper_bound(<i>c</i>.keys.begin(), <i>c</i>.keys.end(), k, <i>compare</i>)</ins><del>ranges::upper_bound(<i>c</i>.keys, k, <i>compare</i>)</del>;
+auto value_it = <i>c</i>.values.begin() + distance(<i>c</i>.keys.begin(), key_it);
+<i>c</i>.keys.emplace(key_it, std::forward&lt;K&gt;(k));
+<i>c</i>.values.emplace(value_it, std::forward&lt;Args&gt;(args)...);
+</pre></blockquote>
+</blockquote>
+</blockquote>
+
+</li>
+
+</ol>
+</resolution>
+
+</issue>