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<title>Issue 4067: Inconsistency and potential infinity meta-recursion in std::chrono::zoned_time's constructors</title>
<meta property="og:title" content="Issue 4067: Inconsistency and potential infinity meta-recursion in std::chrono::zoned_time's constructors">
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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>
<h3 id="4067"><a href="lwg-active.html#4067">4067</a>. Inconsistency and potential infinity meta-recursion in <code>std::chrono::zoned_time</code>'s constructors</h3>
<p><b>Section:</b> 29.11.7.2 <a href="https://wg21.link/time.zone.zonedtime.ctor">[time.zone.zonedtime.ctor]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
<b>Submitter:</b> Jiang An <b>Opened:</b> 2024-04-14 <b>Last modified:</b> 2024-04-14</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#time.zone.zonedtime.ctor">issues</a> in [time.zone.zonedtime.ctor].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, there are no requirements on a program-defined <code>std::chrono::zoned_traits</code> specialization.
So a <code>zoned_traits&lt;<i>UserDefinedTzPtr</i>&gt;::locate_zone</code> possibly returns a <code>string_view</code>,
which leads to infinity meta-recursion when checking the constraints of <code>zoned_time</code>'s constructors
that take a <code>string_view</code>.
<p/>
Also, the constructor of <code>zoned_time</code> taking only one <code>string_view</code> is inconsistent with others
and even self-inconsistent. For other constructors taking <code>string_view</code>, it is expected that the return
value of <code>locate_zone</code> is only implicitly converted to a <code>TimeZonePtr</code>. But the return value is
permitted to be convertible to <code>zoned_time</code> or <code>sys_time&lt;Duration&gt;</code> in this constructor.
And given the <code>locate_name</code> function may be an overload set that distinguishes lvalues and rvalues of
<code>string_view</code>, the use of <code>locate_time(string_view{})</code> in the constraints doesn't always reflect
<code>locate_time(name)</code> in the effects as the return types can be different.
</p>


<p id="res-4067"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4971" title=" Working Draft, Programming Languages — C++">N4971</a>.
</p>

<ol>
<li><p>Modify 29.11.7.2 <a href="https://wg21.link/time.zone.zonedtime.ctor">[time.zone.zonedtime.ctor]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> Add the following two paragraphs to the very beginning of subclause
29.11.7.2 <a href="https://wg21.link/time.zone.zonedtime.ctor">[time.zone.zonedtime.ctor]</a>]
</p>
</blockquote>

<blockquote>
<p>
<ins>-?- Let <code><i>locatable-as-tzptr</i></code> be <code>true</code> if given an lvalue <code>sv</code> of type
<code>string_view</code>, <code>traits::locate_zone(sv)</code> is well-formed and <code>TimeZonePtr</code> is implicitly
convertible from its return type, and <code>false</code> otherwise. Only the validity of the immediate context
of the invocation and conversion is considered.</ins>
<p/>
<ins>-?- In every constructor that takes a <code>string_view</code> parameter <code>name</code>, let
<code><i>converted-tzptr</i></code> be a variable of type <code>TimeZonePtr</code> that is copy-initialized
from <code>traits::locate_zone(name)</code>.</ins>
<p/>
[&hellip;]
</p>
<pre>
explicit zoned_time(string_view name);
</pre>
<blockquote>
<p>
-7- <i>Constraints:</i> <del><code>traits::locate_zone(string_view{})</code> is a well-formed expression and
<code>zoned_time</code> is constructible from the return type of <code>traits::locate_zone(string_view{})</code></del>
<ins><code><i>locatable-as-tzptr</i></code> is <code>true</code></ins>.
<p/>
-8- <i>Effects:</i> Initializes <code>zone_</code> with <code>traits::locate_zone(name)</code> and default
constructs <code>tp_</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
zoned_time(string_view name, const sys_time&lt;Duration&gt;&amp; st);
</pre>
<blockquote>
<p>
-13- <i>Constraints:</i> <del><code>zoned_time</code> is constructible from the return type of
<code>traits::locate_zone(name)</code> and <code>st</code></del><ins><code><i>locatable-as-tzptr</i></code> is
<code>true</code></ins>.
<p/>
-14- <i>Effects:</i> Equivalent to construction with
<code>{<del>traits::locate_zone(name)</del><ins>std::move(<i>converted-tzptr</i>)</ins>, st}</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
zoned_time(string_view name, const local_time&lt;Duration&gt;&amp; tp);
</pre>
<blockquote>
<p>
-18- <i>Constraints:</i> <ins><code><i>locatable-as-tzptr</i></code> is <code>true</code> and</ins>
<code>zoned_time</code> is constructible from <del>the return type of
<code>traits::locate_zone(name)</code></del><ins><code>TimeZonePtr</code></ins> and <code>tp</code>.
<p/>
-19- <i>Effects:</i> Equivalent to construction with
<code>{<del>traits::locate_zone(name)</del><ins>std::move(<i>converted-tzptr</i>)</ins>, tp}</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
zoned_time(string_view name, const local_time&lt;Duration&gt;&amp; tp, choose c);
</pre>
<blockquote>
<p>
-23- <i>Constraints:</i> <ins><code><i>locatable-as-tzptr</i></code> is <code>true</code> and</ins>
<code>zoned_time</code> is constructible from <del>the return type of
<code>traits::locate_zone(name)</code></del><ins><code>TimeZonePtr</code></ins>,
<code>local_time&lt;Duration&gt;</code>, and <code>choose</code>.
<p/>
-24- <i>Effects:</i> Equivalent to construction with
<code>{<del>traits::locate_zone(name)</del><ins>std::move(<i>converted-tzptr</i>)</ins>, tp, c}</code>.
</p>
</blockquote>
<p>
[&hellip;]
</p>
<pre>
template&lt;class Duration2, class TimeZonePtr2&gt;
zoned_time(string_view name, const zoned_time&lt;Duration2, TimeZonePtr2&gt;&amp; y);
</pre>
<blockquote>
<p>
-32- <i>Constraints:</i> <ins><code><i>locatable-as-tzptr</i></code> is <code>true</code> and</ins>
<code>zoned_time</code> is constructible from <del>the return type of
<code>traits::locate_zone(name)</code></del><ins><code>TimeZonePtr</code></ins>
and the type <code>zoned_time&lt;Duration2, TimeZonePtr2&gt;</code>.
<p/>
-33- <i>Effects:</i> Equivalent to construction with
<code>{<del>traits::locate_zone(name)</del><ins>std::move(<i>converted-tzptr</i>)</ins>, y}</code>.
</p>
</blockquote>
<pre>
template&lt;class Duration2, class TimeZonePtr2&gt;
zoned_time(string_view name, const zoned_time&lt;Duration2, TimeZonePtr2&gt;&amp; y, choose c);
</pre>
<blockquote>
<p>
-34- <i>Constraints:</i> <ins><code><i>locatable-as-tzptr</i></code> is <code>true</code> and</ins>
<code>zoned_time</code> is constructible from <del>the return type of
<code>traits::locate_zone(name)</code></del><ins><code>TimeZonePtr</code></ins>, the type
<code>zoned_time&lt;Duration2, TimeZonePtr2&gt;</code>, and the type <code>choose</code>.
<p/>
-35- <i>Effects:</i> Equivalent to construction with
<code>{<del>traits::locate_zone(name)</del><ins>std::move(<i>converted-tzptr</i>)</ins>, y, c}</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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