New issue from Marek Polacek: "Add consteval to std::meta::exception defaulted member functions"

Author: Dani-Hub

xml/issue4494.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4494" status="New">
+<title>Add `consteval` to `std::meta::exception` defaulted member functions</title>
+<section><sref ref="[meta.reflection.exception]"/></section>
+<submitter>Marek Polacek</submitter>
+<date>16 Dec 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+<a href="https://wg21.link/cwg3115">CWG 3115</a> states that every function of consteval-only type shall be an
+immediate function.  This caused a problem for `std::meta::exception::what()` which was marked `constexpr` 
+but not `consteval`. To be able to mark it `consteval`, we had to tweak the rules about overriding by a consteval
+virtual function (<a href="https://wg21.link/cwg3117">CWG 3117</a> ).
+<p/>
+But <sref ref="[meta.reflection.exception]"/> still defines `std::meta::exception` such that it contains these 
+defaulted special member functions:
+</p>
+<blockquote><pre>
+exception(const exception&amp;) = default;
+exception(exception&amp;&amp;) = default;
+
+exception&amp; operator=(const exception&amp;) = default;
+exception&amp; operator=(exception&amp;&amp;) = default;
+</pre></blockquote>
+<p>
+which aren't `consteval` (and since they're not templates, they won't be
+promoted to `consteval` as per <paper num="P2564"/>). I propose to make the four functions
+`consteval`:
+</p>
+<blockquote><pre>
+<ins>consteval</ins> exception(const exception&amp;) = default;
+<ins>consteval</ins> exception(exception&amp;&amp;) = default;
+
+<ins>consteval</ins> exception&amp; operator=(const exception&amp;) = default;
+<ins>consteval</ins> exception&amp; operator=(exception&amp;&amp;) = default;
+</pre></blockquote>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5032"/>.
+</p>
+
+<ol>
+<li><p>Modify <sref ref="[meta.reflection.exception]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+namespace std::meta {
+  class exception : public std::exception {
+  private:
+    optional&lt;string&gt; <i>what_</i>;  <i>// exposition only</i>
+    u8string <i>u8what_</i>;        <i>// exposition only</i>
+    info <i>from_</i>;              <i>// exposition only</i>
+    source_location <i>where_</i>;  <i>// exposition only</i>
+  public:
+    consteval exception(u8string_view what, info from,
+                        source_location where = source_location::current()) noexcept;
+                        
+    consteval exception(string_view what, info from,
+                        source_location where = source_location::current()) noexcept;
+                        
+    <ins>consteval</ins> exception(const exception&amp;) = default;
+    <ins>consteval</ins> exception(exception&amp;&amp;) = default;
+    
+    <ins>consteval</ins> exception&amp; operator=(const exception&amp;) = default;
+    <ins>consteval</ins> exception&amp; operator=(exception&amp;&amp;) = default;
+    
+    constexpr const char* what() const noexcept override;
+    consteval u8string_view u8what() const noexcept;
+    consteval info from() const noexcept;
+    consteval source_location where() const noexcept;
+  };
+}
+</pre>
+</blockquote>
+</li>
+
+</ol>
+
+</resolution>
+
+</issue>