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<title>Issue 3425: condition_variable_any fails to constrain its Lock parameters</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++23">C++23</a> status.</em></p>
<h3 id="3425"><a href="lwg-defects.html#3425">3425</a>. <code>condition_variable_any</code> fails to constrain its <code>Lock</code> parameters</h3>
<p><b>Section:</b> 32.7.5 <a href="https://wg21.link/thread.condition.condvarany">[thread.condition.condvarany]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-04-04 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#thread.condition.condvarany">issues</a> in [thread.condition.condvarany].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
32.7.5 <a href="https://wg21.link/thread.condition.condvarany">[thread.condition.condvarany]</a>/1 says "A <code>Lock</code> type shall meet the <i>Cpp17BasicLockable</i> 
requirements (32.2.5.2 <a href="https://wg21.link/thread.req.lockable.basic">[thread.req.lockable.basic]</a>).", which is fine, but it notably doesn't require anything 
to be a <code>Lock</code> type or meet the requirements of a <code>Lock</code> type. Given that every member template of 
<code>condition_variable_any</code> has a template parameter named <code>Lock</code>, the intent is clearly to impose a 
requirement on the template arguments supplied for those parameters but the use of code font for "<code>Lock</code>" 
in the definition of "<code>Lock</code> type" is a bit subtle to establish that connection. We should specify this 
more clearly.
</p>

<strong>Previous resolution [SUPERSEDED]:</strong>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 32.7.5 <a href="https://wg21.link/thread.condition.condvarany">[thread.condition.condvarany]</a> as indicated:</p>

<blockquote>
<p>
-1- <del>A <code>Lock</code> type</del><ins>Template arguments for template parameters of member templates of 
<code>conditional_variable_any</code> named <code>Lock</code></ins> shall meet the <i>Cpp17BasicLockable</i> 
requirements (32.2.5.2 <a href="https://wg21.link/thread.req.lockable.basic">[thread.req.lockable.basic]</a>). 
[<i>Note:</i> All of the standard mutex types meet this requirement. If a <del><code>Lock</code></del> type other 
than one of the standard mutex types or a <code>unique_lock</code> wrapper for a standard mutex type is used with 
<code>condition_variable_any</code>, the user should ensure that any necessary synchronization is in place with 
respect to the predicate associated with the <code>condition_variable_any</code> instance. &mdash; <i>end note</i>]
</p>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2020-04-06; Tim improves wording]</i></p>


<p><i>[2020-04-11 Issue Prioritization]</i></p>
 
<p>
Status set to Tentatively Ready after seven positive votes on the reflector.
</p>
<p><i>[2020-11-09 Approved In November virtual meeting. Status changed: Tentatively Ready &rarr; WP.]</i></p>



<p id="res-3425"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 32.7.5 <a href="https://wg21.link/thread.condition.condvarany">[thread.condition.condvarany]</a> as indicated:</p>

<blockquote>
<p>
-1- <del>A <code>Lock</code> type</del><ins>In this subclause 32.7.5 <a href="https://wg21.link/thread.condition.condvarany">[thread.condition.condvarany]</a>, template 
arguments for template parameters named <code>Lock</code></ins> shall meet the <i>Cpp17BasicLockable</i> 
requirements (32.2.5.2 <a href="https://wg21.link/thread.req.lockable.basic">[thread.req.lockable.basic]</a>). 
[<i>Note:</i> All of the standard mutex types meet this requirement. If a <del><code>Lock</code></del> type other 
than one of the standard mutex types or a <code>unique_lock</code> wrapper for a standard mutex type is used with 
<code>condition_variable_any</code>, the user should ensure that any necessary synchronization is in place with 
respect to the predicate associated with the <code>condition_variable_any</code> instance. &mdash; <i>end note</i>]
</p>
</blockquote>
</li>
</ol>




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