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<title>Issue 4257: Stream insertion for chrono::local_time should be constrained</title>
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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#Ready">Tentatively Ready</a> status.</em></p>
<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>
<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#Ready">Tentatively Ready</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2025-05-16 <b>Last modified:</b> 2025-08-26</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Tentatively Ready">Tentatively Ready</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Stream insertion for <code class='backtick'>chrono::local_time</code> is defined in terms of conversion to
<code class='backtick'>chrono::sys_time</code>, but not all <code class='backtick'>chrono::sys_time</code> specializations
can be inserted into an ostream, because one of the overloads is
constrained and the other requires convertibility to <code class='backtick'>chrono::sys_days</code>
(see 30.7.2.3 <a href="https://wg21.link/time.clock.system.nonmembers">[time.clock.system.nonmembers]</a>).
</p>
<p>
This means the following code fails to compile:
<pre><code>
#include &lt;iostream&gt;
#include &lt;chrono&gt;

template&lt;typename T&gt;
concept ostream_insertable = requires (std::ostream&amp; o, const T&amp; t) { o &lt;&lt; t; };

using D = std::chrono::duration&lt;double&gt;;

int main() {
  if constexpr (ostream_insertable&lt;std::chrono::sys_time&lt;D&gt;&gt;)
    std::cout &lt;&lt; std::chrono::sys_time&lt;D&gt;{};
  if constexpr (ostream_insertable&lt;std::chrono::local_time&lt;D&gt;&gt;)
    std::cout &lt;&lt; std::chrono::local_time&lt;D&gt;{}; // <b>FAIL</b>
}
</code></pre>
The first condition is false, because there's no overload that's suitable.
The second is true, because the <code>operator&lt;&lt;</code> overload for
<code class='backtick'>chrono::local_time</code> isn't constrained and so insertion appears to be valid.
But actually trying to use it is ill-formed, because it tries to convert the
<code>local_time&lt;D&gt;</code> to a <code>sys_time&lt;D&gt;</code>
and then insert that, which isn't valid.
</p>

<p><i>[2025-08-21; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after six votes in favour during reflector poll.
</p>



<p id="res-4257"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5008" title=" Working Draft, Programming Languages — C++">N5008</a>.
</p>

<ol>
<li><p>Modify 30.7.9 <a href="https://wg21.link/time.clock.local">[time.clock.local]</a> as indicated:</p>

<blockquote>
<pre><code>
template&lt;class charT, class traits, class Duration&gt;
  basic_ostream&lt;charT, traits&gt;&amp;
    operator&lt;&lt;(basic_ostream&lt;charT, traits&gt;&amp; os, const local_time&lt;Duration&gt;&amp; lt);
</code></pre>
<p><ins>-?- <i>Constraints</i>:
<code>os &lt;&lt; sys_time&lt;Duration&gt;{lt.time_since_epoch()}</code>
is a valid expression.</ins>
</p>
<p>-2- <i>Effects</i>:
<pre><code> os &lt;&lt; sys_time&lt;Duration&gt;{lt.time_since_epoch()};
</code></pre>
</p>
<p>-3- <i>Returns</i>: <code class='backtick'>os</code>.</p>
</blockquote>
</li>
</ol>






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