New issue from Jiang An: "Static data members of ctype_base are not y…

…et required to be usable in constant expressions"

xml/issue4037.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4037" status="New">
+<title>Static data members of <tt>ctype_base</tt> are not yet required to be usable in constant expressions</title>
+<section><sref ref="[category.ctype.general]"/></section>
+<submitter>Jiang An</submitter>
+<date>12 Jan 2024</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+It may be desired that static data members <tt>ctype_base</tt> are "real constants", i.e. usable in constant expressions. 
+However, this is not strictly required because <tt>mask</tt> is only required to be a bitmask type that can be a class type, 
+which makes the plain <tt>const</tt> potentially insufficient.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N4971"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[category.ctype.general]"/> as indicated:</p>
+
+<blockquote>
+<blockquote>
+<pre>
+namespace std {
+  class ctype_base {
+  public:
+    using mask = <i>see below</i>;
+
+    // <i>numeric values are for exposition only.</i>
+    static const<ins>expr</ins> mask space  = 1 &lt;&lt; 0;
+    static const<ins>expr</ins> mask print  = 1 &lt;&lt; 1;
+    static const<ins>expr</ins> mask cntrl  = 1 &lt;&lt; 2;
+    static const<ins>expr</ins> mask upper  = 1 &lt;&lt; 3;
+    static const<ins>expr</ins> mask lower  = 1 &lt;&lt; 4;
+    static const<ins>expr</ins> mask alpha  = 1 &lt;&lt; 5;
+    static const<ins>expr</ins> mask digit  = 1 &lt;&lt; 6;
+    static const<ins>expr</ins> mask punct  = 1 &lt;&lt; 7;
+    static const<ins>expr</ins> mask xdigit = 1 &lt;&lt; 8;
+    static const<ins>expr</ins> mask blank  = 1 &lt;&lt; 9;
+    static const<ins>expr</ins> mask alnum  = alpha | digit;
+    static const<ins>expr</ins> mask graph  = alnum | punct;
+  };
+}
+</pre>
+</blockquote>
+<p>
+-1- The type mask is a bitmask type (<sref ref="[bitmask.types]"/>).
+</p>
+</blockquote>
+
+
+</li>
+
+</ol>
+
+
+</resolution>
+
+</issue>