New issue from Hewill: "subrange should prevent slicing for contiguous iterators"

Author: Dani-Hub

xml/issue4565.xml

@@ -0,0 +1,66 @@
+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4565" status="New">
+<title><code>subrange</code> should prevent slicing for contiguous iterators</title>
+<section>
+<sref ref="[range.subrange.general]"/>
+</section>
+<submitter>Hewill Kang</submitter>
+<date>03 Apr 2026</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+Currently, the <code>subrange</code> uses the following exposition-only concept:
+</p>
+<blockquote><pre>
+template&lt;class From, class To&gt;
+  concept <i>uses-nonqualification-pointer-conversion</i> =      // <i>exposition only</i>
+    is_pointer_v&lt;From&gt; &amp;&amp; is_pointer_v&lt;To&gt; &amp;&amp;
+    !convertible_to&lt;remove_pointer_t&lt;From&gt;(*)[], remove_pointer_t&lt;To&gt;(*)[]&gt;;
+</pre></blockquote>
+<p>
+to disallow derived to base conversions. However, this only applies to pointers.
+</p>
+<p>
+Given that <code>subrange</code> is intended to universally take any iterator-pair to 
+form a view, we may need to generally prevent this in the case of contiguous iterators 
+(<a href="https://godbolt.org/z/oPvvExxaT">demo</a>):
+</p>
+<blockquote><pre>
+struct Base {};
+struct Derived : Base {};
+ranges::subrange&lt;Derived*&gt; sd;
+ranges::subrange&lt;Base*&gt; sb = sd;                        // error
+ranges::subrange&lt;const Base*&gt; sb = views::as_const(sd); // <span style="color:#C80000;font-weight:bold">ok</span>
+</pre></blockquote>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5032"/>.
+</p>
+
+<ol>
+<li>
+<p>Modify <sref ref="[range.subrange.general]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+namespace std::ranges {
+  template&lt;class From, class To&gt;
+    concept <i>uses-nonqualification-pointer-conversion</i> =      // <i>exposition only</i>
+      <del>is_pointer_v</del><ins>contiguous_iterator</ins>&lt;From&gt; &amp;&amp; <del>is_pointer_v</del><ins>contiguous_iterator</ins>&lt;To&gt; &amp;&amp;
+      !convertible_to&lt;<del>remove_pointer_t</del><ins>remove_reference_t&lt;iter_reference_t</ins>&lt;From&gt;<ins>&gt;</ins>(*)[], 
+                      <del>remove_pointer_t</del><ins>remove_reference_t&lt;iter_reference_t</ins>&lt;To&gt;<ins>&gt;</ins>(*)[]&gt;;
+   [&hellip;]
+}
+</pre>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>