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<!DOCTYPE html>
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<html lang="en">
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<meta charset="utf-8">
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<title>Issue 4223: Deduction guides for maps are mishandling tuples and references</title>
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<meta property="og:title" content="Issue 4223: Deduction guides for maps are mishandling tuples and references">
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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="4223"><a href="lwg-active.html#4223">4223</a>. Deduction guides for maps are mishandling tuples and references</h3>
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<p><b>Section:</b> 23.4.1 <a href="https://wg21.link/associative.general">[associative.general]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Tomasz Kaminski <b>Opened:</b> 2025-03-14 <b>Last modified:</b> 2025-03-15</p>
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<p><b>Priority: </b>Not Prioritized
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</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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The <code class='backtick'>from_range</code> deduction guide for maps currently do not handle ranges of tuple of two elements:
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</p>
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<blockquote><pre>
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std::vector&lt;int&gt; v;
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auto zv = std::views::zip(v, v);
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std::map m4(std::from_range, zv); // <span style="color:red;font-weight:bolder">Ill-formed, no-deduction guide</span>
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</pre></blockquote>
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<p>
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This seems to be result of merge conflict between <a href="https://wg21.link/P2165" title=" Compatibility between tuple, pair and tuple-like objects">P2165</a> (Compatibility between tuple, pair and tuple-like objects)
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and <a href="https://wg21.link/P1206R4" title=" Conversions from ranges to containers">P1206R4</a> (Conversions from ranges to containers): The helper <code><i>range-key-type</i></code> and
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<code><i>range-mapped-type</i></code> aliases introduced by the later use the old formulation of <code class='backtick'>::first_type</code>,
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<code class='backtick'>::second::type</code> instead of <code class='backtick'>tuple_element</code>.
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<p/>
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Furthermore, both iterator and range deduction guides do not correctly handle iterators with a pair of references as
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value types, and deduce key or value type as reference, which is ill-formed:
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</p>
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<blockquote><pre>
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std::flat_map&lt;int, float&gt; fm; // iterator value_type is pair&lt;int, float&gt;
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std::map m1(fm.begin(), fm.end()); // OK, deduces std::map&lt;int, float&gt;
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auto tv = fm | std::views::transform(std::identity{}); // iterator value value_type is pair&lt;int const&amp;, float const&amp;&gt;
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std::map m3(tv.begin(), tv.end()); // <span style="color:red;font-weight:bolder">Ill-formed, deduces std::map&lt;int const&amp;, float&amp;&gt;</span>
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</pre></blockquote>
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<p id="res-4223"><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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<blockquote class="note">
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<p>
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[<i>Drafting note</i>: The proposed change also strips <code class='backtick'>const</code> from the value type of the <code class='backtick'>map</code>,
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changing the behavior of previously working code:
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</p>
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<blockquote><pre>
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std::pair&lt;int const, float const&gt; tp[2];
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std::map m(std::begin(tp), std::end(tp)); // Was std::map&lt;int, float const&gt;, now std::map&lt;int, float&gt;
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</pre></blockquote>
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]
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</blockquote>
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<ol>
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<li><p>Modify 23.4.1 <a href="https://wg21.link/associative.general">[associative.general]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class InputIterator&gt;
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using <i>iter-value-type</i> =
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typename iterator_traits&lt;InputIterator&gt;::value_type; // <i>exposition only</i>
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template&lt;class InputIterator&gt;
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using <i>iter-key-type</i> = <del>remove_const_t</del><ins>remove_cvref_t</ins>&lt;
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tuple_element_t&lt;0, <i>iter-value-type</i>&lt;InputIterator&gt;&gt;&gt;; // <i>exposition only</i>
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template&lt;class InputIterator&gt;
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using <i>iter-mapped-type</i> = <ins>remove_cvref_t&lt;</ins>
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tuple_element_t&lt;1, <i>iter-value-type</i>&lt;InputIterator&gt;&gt;<ins>&gt;</ins>; // <i>exposition only</i>
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template&lt;class InputIterator&gt;
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using <i>iter-to-alloc-type</i> = pair&lt;
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add_const_t&lt;
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<del>tuple_element_t&lt;0, <i>iter-value-type</i>&lt;InputIterator&gt;&gt;</del>
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<ins><i>iter-key-type</i>&lt;InputIterator&gt;</ins>
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&gt;,
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<del>tuple_element_t&lt;1, <i>iter-value-type</i>&lt;InputIterator&gt;&gt;</del>
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<ins><i>iter-mapped-type</i>&lt;InputIterator&gt;</ins>
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&gt;; // <i>exposition only</i>
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template&lt;ranges::input_range Range&gt;
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using <i>range-key-type</i> =
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<del>remove_const_t&lt;typename ranges::range_value_t&lt;Range&gt;::first_type&gt;</del>
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<ins>remove_cvref_t&lt;tuple_element_t&lt;0, ranges::range_value_t&lt;Range&gt;&gt;&gt;</ins>; // <i>exposition only</i>
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template&lt;ranges::input_range Range&gt;
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using <i>range-mapped-type</i> =
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<del>typename ranges::range_value_t&lt;Range&gt;::second_type</del>
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<ins>remove_cvref_t&lt;tuple_element_t&lt;1, ranges::range_value_t&lt;Range&gt;&gt;&gt;</ins>; // <i>exposition only</i>
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template&lt;ranges::input_range Range&gt;
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using <i>range-to-alloc-type</i> =
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pair&lt;add_const_t&lt;
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<del>typename ranges::range_value_t&lt;Range&gt;::first_type</del>
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<ins><i>range-key-type</i>&lt;Range&gt;</ins>
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&gt;,
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<del>typename ranges::range_value_t&lt;Range&gt;::second_type</del>
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<ins><i>range-mapped-type</i>&lt;Range&gt;</ins>
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&gt;; // <i>exposition only</i>
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</pre>
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