New issue from Jiang An: "Integer-class types should be formattable"

Author: Dani-Hub

xml/issue4284.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4284" status="New">
+<title>Integer-class types should be formattable</title>
+<section>
+<sref ref="[format.formatter.spec]"/>
+</section>
+<submitter>Jiang An</submitter>
+<date>27 Jun 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+The following program doesn't compile with MSVC STL (<a href="https://godbolt.org/z/aazK6bhqM">Godbolt link</a>).
+</p>
+<blockquote><pre>
+#include &lt;format&gt;
+#include &lt;ranges&gt;
+
+int main() {
+  auto iv1 = std::views::iota(42ull, 1729ull);
+  auto iv2 = std::views::iota(iv1.begin(), iv1.end());
+  std::format("{}", iv2.size()); // <span style="color:red;font-weight:bolder">Error</span>
+}
+</pre></blockquote>
+<p>
+In MSVC STL, the type of `iv2.size()` is an integer-class type, and the standard hasn't guaranteed there're 
+enabled formatter specializations for integer-class types. As a result, this program is not guaranteed to 
+be well-formed.
+<p/>
+Perhaps we should require enabled `std::formatter` specializations for integer-class types to get rid of 
+such uncertainty and inconsistency.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5008"/>.
+</p>
+
+<ol>
+<li><p>Modify <sref ref="[format.formatter.spec]"/> as indicated:</p>
+
+<blockquote>
+<p>
+-2- [&hellip;] Each header that declares the template
+`formatter` provides the following enabled specializations:
+</p>
+<ol style="list-style-type: none">
+<li><p>(2.1) &mdash; [&hellip;]</p></li>
+<li><p>(2.2) &mdash; [&hellip;]</p></li>
+<li><p>(2.3) &mdash; [&hellip;]</p></li>
+<li><p><ins>(2.?) &mdash; For each `charT`, for each cv-unqualified integer-class type 
+(<sref ref="[iterator.concept.winc]"/>) `IntegerClass`, a specialization</ins>
+</p>
+<blockquote><pre>
+<ins>template&lt;&gt; struct formatter&lt;IntegerClass, charT&gt;;</ins>
+</pre></blockquote>
+</li>
+<li><p>(2.4) &mdash; [&hellip;]</p></li>
+</ol>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>