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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 4246: Redundant constraint in range_formatter::format</title>
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<meta property="og:title" content="Issue 4246: Redundant constraint in range_formatter::format">
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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="4246"><a href="lwg-active.html#4246">4246</a>. Redundant constraint in <code class='backtick'>range_formatter::format</code></h3>
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<p><b>Section:</b> 28.5.7.2 <a href="https://wg21.link/format.range.formatter">[format.range.formatter]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-04-18 <b>Last modified:</b> 2025-04-21</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#format.range.formatter">issues</a> in [format.range.formatter].</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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Currently, the signature of <code class='backtick'>range_formatter::format</code> is as follows:
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</p>
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<blockquote><pre>
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template&lt;ranges::input_range R, class FormatContext&gt;
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requires formattable&lt;ranges::range_reference_t&lt;R&gt;, charT&gt; &amp;&amp;
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same_as&lt;remove_cvref_t&lt;ranges::range_reference_t&lt;R&gt;&gt;, T&gt;
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typename FormatContext::iterator
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format(R&amp;&amp; r, FormatContext&amp; ctx) const;
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</pre></blockquote>
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<p>
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which requires that the reference type of the range parameter must be <code class='backtick'>formattable</code>,
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and such type must be exactly <code class='backtick'>T</code> after removing the <i>cvref</i>-qualifiers.
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</p>
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<p>
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However, satisfying the latter always implies satisfying the former, as the <code class='backtick'>range_formatter</code> class
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already requires that <code class='backtick'>T</code> must be <code class='backtick'>formattable</code>.
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</p>
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<p>
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There is no need to perform a redundant check here.
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</p>
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<p id="res-4246"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5008">N5008</a>.
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</p>
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<ol>
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<li><p>Modify 28.5.7.2 <a href="https://wg21.link/format.range.formatter">[format.range.formatter]</a> as indicated:</p>
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<blockquote>
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<blockquote>
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<pre>
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namespace std {
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template&lt;class T, class charT = char&gt;
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requires same_as&lt;remove_cvref_t&lt;T>, T> &amp;&amp; formattable&lt;T, charT&gt;
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class range_formatter {
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[&hellip;]
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template&lt;ranges::input_range R, class FormatContext&gt;
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requires <del>formattable&lt;ranges::range_reference_t&lt;R&gt;, charT&gt; &amp;&amp;</del>
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same_as&lt;remove_cvref_t&lt;ranges::range_reference_t&lt;R&gt;&gt;, T&gt;
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typename FormatContext::iterator
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format(R&amp;&amp; r, FormatContext&amp; ctx) const;
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};
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}
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</pre>
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</blockquote>
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[&hellip;]
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<pre>
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template&lt;ranges::input_range R, class FormatContext&gt;
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requires <del>formattable&lt;ranges::range_reference_t&lt;R&gt;, charT&gt; &amp;&amp;</del>
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same_as&lt;remove_cvref_t&lt;ranges::range_reference_t&lt;R&gt;&gt;, T&gt;
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typename FormatContext::iterator
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format(R&amp;&amp; r, FormatContext&amp; ctx) const;
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</pre>
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<blockquote>
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<p>
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-11- <i>Effects</i>: Writes the following into <code>ctx.out()</code>, adjusted according to the
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<i>range-format-spec</i>:
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</p>
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</blockquote>
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</blockquote>
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</li>
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</body>
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</html>

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