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<title>Issue 3050: Conversion specification problem in chrono::duration constructor</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#C++20">C++20</a> status.</em></p>
<h3 id="3050"><a href="lwg-defects.html#3050">3050</a>. Conversion specification problem in <code>chrono::duration</code> constructor</h3>
<p><b>Section:</b> 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a>, 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Barry Revzin <b>Opened:</b> 2018-01-22 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.duration.cons">issues</a> in [time.duration.cons].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++20">C++20</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The converting constructor in <code>std::chrono::duration</code> is currently specified as
(30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a> p1):
</p>
<blockquote>
<pre>
template&lt;class Rep2&gt;
  constexpr explicit duration(const Rep2&amp; r);
</pre>
<blockquote>
<p>
<i>Remarks:</i> This constructor shall not participate in overload resolution unless <code>Rep2</code> is
implicitly convertible to <code>rep</code> and [&hellip;]
</p>
</blockquote>
</blockquote>
<p>
But the parameter is of type <code>Rep2 const</code>, so we should check that <code>Rep2 const</code> is
implicitly convertible to <code>rep</code>, not just <code>Rep2</code>. This means that for a type like:
</p>
<blockquote>
<pre>
struct X { operator int64_t() /* not const */; };
</pre>
</blockquote>
<p>
<code>std::is_constructible_v&lt;std::chrono::seconds, X&gt;</code> is <code>true</code>, but actual
construction will fail to compile.
<p/>
Further analysis of sub-clause 30.5 <a href="https://wg21.link/time.duration">[time.duration]</a> revealed similar specification problems in
some other functions.
</p>

<p><i>[2018-06 Rapperswil Thursday issues processing]</i></p>

<p>P3; Status to Open</p>

<p><i>[2018-11 San Diego Thursday night issue processing]</i></p>

<p>Jonathan to provide updated wording; the underlying text has changed.</p>

<p><i>[2018-12-05 Jonathan provides new wording]</i></p>

<p>In San Diego Geoff noticed that the current WP does not use <code>CR</code>.
Jonathan provides new wording consistent with the
<a href="https://github.com/cplusplus/draft/pull/1985">editorial changes</a>
that removed <code>CR</code>.</p>

<strong>Previous resolution [SUPERSEDED]:</strong>

<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4713">N4713</a>.</p>

<ol>
<li><p>Modify 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep2&gt;
  constexpr explicit duration(const Rep2&amp; r);
</pre>
<blockquote>
<p>
-1- <i>Remarks:</i> This constructor shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, rep&gt;</code> is <code>true</code></ins><del><code>Rep2</code>
is implicitly convertible to <code>rep</code></del> and
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; <code>treat_as_floating_point_v&lt;rep&gt;</code> is <code>true</code> or</p></li>
<li><p>(1.2) &mdash; <code>treat_as_floating_point_v&lt;Rep2&gt;</code> is <code>false</code>.</p></li>
</ol>
[&hellip;]
<p/>
-2- <i>Effects:</i> Constructs an object of type <code>duration</code>.
<p/>
-3- <i>Postconditions:</i> <code>count() == static_cast&lt;rep&gt;(r)</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-4- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, CR(Rep1, Rep2)&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>CR(Rep1, Rep2)</code></del>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Rep2, class Period&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const Rep1&amp; s, const duration&lt;Rep2, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
-6- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep1&amp;, CR(Rep1, Rep2)&gt;</code> is
<code>true</code></ins><del><code>Rep1</code> is implicitly convertible to <code>CR(Rep1, Rep2)</code></del>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator/(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-8- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, CR(Rep1, Rep2)&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>CR(Rep1, Rep2)</code></del>
and <code>Rep2</code> is not a specialization of <code>duration</code>.
</p>
</blockquote>
[&hellip;]
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator%(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-11- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, CR(Rep1, Rep2)&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>CR(Rep1, Rep2)</code></del>
and <code>Rep2</code> is not a specialization of <code>duration</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>This wording is relative to <a href="https://wg21.link/n4791">N4791</a>.</p>

<ol>
<li><p>Modify 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep2&gt;
  constexpr explicit duration(const Rep2&amp; r);
</pre>
<blockquote>
<p>
-1- <i>Remarks:</i> This constructor shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, rep&gt;</code> is <code>true</code></ins><del><code>Rep2</code>
is implicitly convertible to <code>rep</code></del> and
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; <code>treat_as_floating_point_v&lt;rep&gt;</code> is <code>true</code> or</p></li>
<li><p>(1.2) &mdash; <code>treat_as_floating_point_v&lt;Rep2&gt;</code> is <code>false</code>.</p></li>
</ol>
[&hellip;]
<p/>
-2- <i>Effects:</i> Constructs an object of type <code>duration</code>.
<p/>
-3- <i>Postconditions:</i> <code>count() == static_cast&lt;rep&gt;(r)</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-4- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>common_type_t&lt;Rep1, Rep2&gt;</code></del>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Rep2, class Period&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const Rep1&amp; s, const duration&lt;Rep2, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
-6- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep1&amp;, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is
<code>true</code></ins><del><code>Rep1</code> is implicitly convertible to <code>common_type_t&lt;Rep1, Rep2&gt;</code></del>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator/(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-8- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>common_type_t&lt;Rep1, Rep2&gt;</code></del>
and <code>Rep2</code> is not a specialization of <code>duration</code>.
</p>
</blockquote>
[&hellip;]
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator%(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-11- <i>Remarks:</i> This operator shall not participate in overload resolution unless
<ins><code>is_convertible_v&lt;const Rep2&amp;, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is
<code>true</code></ins><del><code>Rep2</code> is implicitly convertible to <code>common_type_t&lt;Rep1, Rep2&gt;</code></del>
and <code>Rep2</code> is not a specialization of <code>duration</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2020-02-13, Prague]</i></p>

<p>
Rebase to most recent working draft
</p>

<p><i>[2020-02 Status to Immediate on Thursday night in Prague.]</i></p>



<p id="res-3050"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4849">N4849</a>.</p>

<ol>
<li><p>Modify 30.5.2 <a href="https://wg21.link/time.duration.cons">[time.duration.cons]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep2&gt;
  constexpr explicit duration(const Rep2&amp; r);
</pre>
<blockquote>
<p>
-1- <i>Constraints:</i> <code>is_convertible_v&lt;<ins>const</ins> Rep2<ins>&amp;</ins>, rep&gt;</code> is <code>true</code> and
<ol style="list-style-type: none">
<li><p>(1.1) &mdash; <code>treat_as_floating_point_v&lt;rep&gt;</code> is <code>true</code> or</p></li>
<li><p>(1.2) &mdash; <code>treat_as_floating_point_v&lt;Rep2&gt;</code> is <code>false</code>.</p></li>
</ol>
[&hellip;]
<p/>
-2- <i>Postconditions:</i> <code>count() == static_cast&lt;rep&gt;(r)</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 30.5.6 <a href="https://wg21.link/time.duration.nonmember">[time.duration.nonmember]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-4- <i>Constraints:</i> <code>is_convertible_v&lt;<ins>const</ins> Rep2<ins>&amp;</ins>, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Rep2, class Period&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator*(const Rep1&amp; s, const duration&lt;Rep2, Period&gt;&amp; d);
</pre>
<blockquote>
<p>
-6- <i>Constraints:</i> <code>is_convertible_v&lt;<ins>const</ins> Rep1<ins>&amp;</ins>, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator/(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-8- <i>Constraints:</i> <code>is_convertible_v&lt;<ins>const</ins> Rep2<ins>&amp;</ins>, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> 
is <code>true</code> and <code>Rep2</code> is not a specialization of <code>duration</code>.
</p>
</blockquote>
[&hellip;]
<pre>
template&lt;class Rep1, class Period, class Rep2&gt;
  constexpr duration&lt;common_type_t&lt;Rep1, Rep2&gt;, Period&gt;
    operator%(const duration&lt;Rep1, Period&gt;&amp; d, const Rep2&amp; s);
</pre>
<blockquote>
<p>
-12- <i>Constraints:</i> <code>is_convertible_v&lt;<ins>const</ins> Rep2<ins>&amp;</ins>, common_type_t&lt;Rep1, Rep2&gt;&gt;</code> 
is <code>true</code> and <code>Rep2</code> is not a specialization of <code>duration</code>.
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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