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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4220: join_view incorrectly stores inner range</title>
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<meta property="og:title" content="Issue 4220: join_view incorrectly stores inner range">
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<meta property="og:description" content="C++ library issue. Status: New">
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#New">New</a> status.</em></p>
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<h3 id="4220"><a href="lwg-active.html#4220">4220</a>. <code>join_view</code> incorrectly stores inner range</h3>
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<p><b>Section:</b> 25.7.14.2 <a href="https://wg21.link/range.join.view">[range.join.view]</a>, 25.7.15.2 <a href="https://wg21.link/range.join.with.view">[range.join.with.view]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-03-06 <b>Last modified:</b> 2025-03-09</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View all other</b> <a href="lwg-index.html#range.join.view">issues</a> in [range.join.view].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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When the inner range is a prvalue, <code>join_view</code> removes its <i>cv</i>-qualifiers
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and stores it in the <code><i>propagating-cache</i></code>, which is not quite right as the inner range may
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only be const-iterable (<a href="https://godbolt.org/z/qY1bhExzG">demo</a>):
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</p>
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<blockquote><pre>
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#include &lt;ranges&gt;
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struct R {
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int* begin() = delete;
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int* end() = delete;
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const int* begin() const;
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const int* end() const;
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};
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int main() {
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auto r = std::views::iota(0, 5)
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| std::views::transform([](int) -> const R { return {}; })
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| std::views::join;
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auto b = r.begin(); // <span style="color:red;font-weight:bolder">hard error</span>
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}
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</pre></blockquote>
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<p>
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The proposed resolution preserves the inner range's original qualifiers, which is consistent with how
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<code>cache_latest_view</code> stores the reference when it is a prvalue.
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The same goes for <code>join_with_view</code>.
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</p>
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<p id="res-4220"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5001">N5001</a>.
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</p>
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<ol>
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<li><p>Modify 25.7.14.2 <a href="https://wg21.link/range.join.view">[range.join.view]</a> as indicated:</p>
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<blockquote>
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<pre>
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namespace std::ranges {
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template&lt;input_range V&gt;
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requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt;&gt;
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class join_view : public view_interface&lt;join_view&lt;V&gt;&gt; {
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private:
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using <i>InnerRng</i> = range_reference_t&lt;V&gt;; // <i>exposition only</i>
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[&hellip;]
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<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>
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// <i>if is_reference_v&lt;<i>InnerRng</i>&gt; is false</i>
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public:
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[&hellip;]
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};
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[&hellip;]
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}
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</pre>
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</blockquote>
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</li>
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<li><p>Modify 25.7.15.2 <a href="https://wg21.link/range.join.with.view">[range.join.with.view]</a> as indicated:</p>
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<blockquote>
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<pre>
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namespace std::ranges {
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[&hellip;]
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template&lt;input_range V, forward_range Pattern&gt;
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requires view&lt;V&gt; &amp;&amp; input_range&lt;range_reference_t&lt;V&gt;&gt;
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&amp;&amp; view&lt;Pattern&gt;
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&amp;&amp; <i>concatable</i>&lt;range_reference_t&lt;V&gt;, Pattern&gt;
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class join_with_view : public view_interface&lt;join_with_view&lt;V, Pattern&gt;&gt; {
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using <i>InnerRng</i> = range_reference_t&lt;V&gt;; // <i>exposition only</i>
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[&hellip;]
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<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>
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// <i>if is_reference_v&lt;<i>InnerRng</i>&gt; is false</i>
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[&hellip;]
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public:
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[&hellip;]
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};
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[&hellip;]
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}
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</pre>
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</blockquote>
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</li>
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</ol>
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</body>
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</html>

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