New issue from Hewill: "join_view incorrectly stores inner range"

Author: Dani-Hub

xml/issue4220.xml

@@ -0,0 +1,103 @@
+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4220" status="New">
+<title><code>join_view</code> incorrectly stores inner range</title>
+<section>
+<sref ref="[range.join.view]"/><sref ref="[range.join.with.view]"/>
+</section>
+<submitter>Hewill Kang</submitter>
+<date>06 Mar 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+When the inner range is a prvalue, <code>join_view</code> removes its <i>cv</i>-qualifiers
+and stores it in the <code><i>propagating-cache</i></code>, which is not quite right as the inner range may
+only be const-iterable (<a href="https://godbolt.org/z/qY1bhExzG">demo</a>):
+</p>
+<blockquote><pre>
+#include &lt;ranges&gt;
+
+struct R {
+  int* begin() = delete;
+  int* end() = delete;
+  const int* begin() const;
+  const int* end() const;
+};
+
+int main() {
+  auto r = std::views::iota(0, 5)
+         | std::views::transform([](int) -> const R { return {}; })
+         | std::views::join;
+  auto b = r.begin(); // <span style="color:red;font-weight:bolder">hard error</span>
+}
+</pre></blockquote>
+<p>
+The proposed resolution preserves the inner range's original qualifiers, which is consistent with how
+<code>cache_latest_view</code> stores the reference when it is a prvalue.
+The same goes for <code>join_with_view</code>.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to <paper num="N5001"/>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[range.join.view]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+namespace std::ranges {
+  template&lt;input_range V&gt;
+    requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt;&gt;
+  class join_view : public view_interface&lt;join_view&lt;V&gt;&gt; {
+  private:
+    using <i>InnerRng</i> = range_reference_t&lt;V&gt;;                  // <i>exposition only</i>
+    [&hellip;]
+    <i>non-propagating-cache</i>&lt;<del>remove_cv_t&lt;</del><i>InnerRng</i><del>&gt;</del>&gt; <i>inner_</i>;    // <i>exposition only, present only</i>
+                                                            // <i>if is_reference_v&lt;<i>InnerRng</i>&gt; is false</i>
+
+  public:
+    [&hellip;]
+  };
+  [&hellip;]
+}
+</pre>
+</blockquote>
+</li>
+
+<li><p>Modify <sref ref="[range.join.with.view]"/> as indicated:</p>
+
+<blockquote>
+<pre>
+namespace std::ranges {
+  [&hellip;]
+  template&lt;input_range V, forward_range Pattern&gt;
+    requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt;
+          &amp;&amp; view&lt;Pattern&gt;
+          &amp;&amp; <i>concatable</i>&lt;range_reference_t&lt;V&gt;, Pattern&gt;
+  class join_with_view : public view_interface&lt;join_with_view&lt;V, Pattern&gt;&gt; {
+    using <i>InnerRng</i> = range_reference_t&lt;V&gt;;                 // <i>exposition only</i>
+    [&hellip;]
+    <i>non-propagating-cache</i>&lt;<del>remove_cv_t&lt;</del><i>InnerRng</i><del>&gt;</del>&gt; <i>inner_</i>;   // <i>exposition only, present only</i>
+                                                           // <i>if is_reference_v&lt;<i>InnerRng</i>&gt; is false</i>
+
+    [&hellip;]
+  public:
+    [&hellip;]
+  };
+  [&hellip;]
+}
+
+</pre>
+</blockquote>
+</li>
+</ol>
+
+</resolution>
+
+</issue>