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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 4257: Stream insertion for chrono::local_time should be constrained</title>
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<meta property="og:title" content="Issue 4257: Stream insertion for chrono::local_time should be constrained">
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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="4257"><a href="lwg-active.html#4257">4257</a>. Stream insertion for <code class='backtick'>chrono::local_time</code> should be constrained</h3>
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<p><b>Section:</b> 30.7.9 <a href="https://wg21.link/time.clock.local">[time.clock.local]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-05-16 <b>Last modified:</b> 2025-05-16</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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Stream insertion for <code class='backtick'>chrono::local_time</code> is defined in terms of conversion to
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<code class='backtick'>chrono::sys_time</code>, but not all <code class='backtick'>chrono::sys_time</code> specializations
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can be inserted into an ostream, because one of the overloads is
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constrained and the other requires convertibility to <code class='backtick'>chrono::sys_days</code>
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(see 30.7.2.3 <a href="https://wg21.link/time.clock.system.nonmembers">[time.clock.system.nonmembers]</a>).
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</p>
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<p>
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This means the following code fails to compile:
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<pre><code>
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#include &lt;iostream&gt;
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#include &lt;chrono&gt;
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template&lt;typename T&gt;
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concept ostream_insertable = requires (std::ostream&amp; o, const T&amp; t) { o &lt;&lt; t; };
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using D = std::chrono::duration&lt;double&gt;;
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int main() {
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if constexpr (ostream_insertable&lt;std::chrono::sys_time&lt;D&gt;&gt;)
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std::cout &lt;&lt; std::chrono::sys_time&lt;D&gt;{};
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if constexpr (ostream_insertable&lt;std::chrono::local_time&lt;D&gt;&gt;)
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std::cout &lt;&lt; std::chrono::local_time&lt;D&gt;{}; // <b>FAIL</b>
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}
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</code></pre>
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The first condition is false, because there's no overload that's suitable.
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The second is true, because the <code>operator&lt;&lt;</code> overload for
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<code class='backtick'>chrono::local_time</code> isn't constrained and so insertion appears to be valid.
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But actually trying to use it is ill-formed, because it tries to convert the
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<code>local_time&lt;D&gt;</code> to a <code>sys_time&lt;D&gt;</code>
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and then insert that, which isn't valid.
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</p>
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<p id="res-4257"><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 30.7.9 <a href="https://wg21.link/time.clock.local">[time.clock.local]</a> as indicated:</p>
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<blockquote>
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<pre><code>
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template&lt;class charT, class traits, class Duration&gt;
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basic_ostream&lt;charT, traits&gt;&amp;
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operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp; os, const local_time&lt;Duration&gt;&amp; lt);
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</code></pre>
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<p><ins>-?- <i>Constraints</i>:
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<code>os &lt;&lt; sys_time&lt;Duration&gt;{lt.time_since_epoch()}</code>
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is a valid expression.</ins>
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</p>
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<p>-2- <i>Effects</i>:
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<pre><code> os &lt;&lt; sys_time&lt;Duration&gt;{lt.time_since_epoch()};
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</code></pre>
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</p>
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<p>-3- <i>Returns</i>: <code class='backtick'>os</code>.</p>
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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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