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<title>Issue 2981: Remove redundant deduction guides from standard library</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="2981"><a href="lwg-defects.html#2981">2981</a>. Remove redundant deduction guides from standard library</h3>
<p><b>Section:</b> 22.10.6 <a href="https://wg21.link/refwrap">[refwrap]</a>, 32.6.5.2 <a href="https://wg21.link/thread.lock.guard">[thread.lock.guard]</a>, 32.6.5.3 <a href="https://wg21.link/thread.lock.scoped">[thread.lock.scoped]</a>, 32.6.5.4 <a href="https://wg21.link/thread.lock.unique">[thread.lock.unique]</a>, 32.6.5.5 <a href="https://wg21.link/thread.lock.shared">[thread.lock.shared]</a> <b>Status:</b> <a href="lwg-active.html#C++20">C++20</a>
 <b>Submitter:</b> Mike Spertus <b>Opened:</b> 2017-06-15 <b>Last modified:</b> 2021-02-25</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#refwrap">issues</a> in [refwrap].</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>
There are several deduction guides added to the standard library by <a href="https://wg21.link/p0433r2">P0433R2</a> 
that have no effect probably because LWG had not considered late changes to core wording that automatically adds 
a "copy deduction candidate" (12.2.2.9 <a href="https://wg21.link/over.match.class.deduct">[over.match.class.deduct]</a>) that renders these explicit guides moot. 
</p>

<p><i>[2017-07 Toronto Monday issue prioritization]</i></p>

<p>Priority 0; move to Ready</p>


<p id="res-2981"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4659">N4659</a>.
</p>

<ol>
<li><p>Edit 22.10.6 <a href="https://wg21.link/refwrap">[refwrap]</a>, end of class template <code>reference_wrapper</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
<del>template&lt;class T&gt;
  reference_wrapper(reference_wrapper&lt;T&gt;) -&gt; reference_wrapper&lt;T&gt;;</del>
</pre>
</blockquote>
</li>

<li><p>Edit 32.6.5.2 <a href="https://wg21.link/thread.lock.guard">[thread.lock.guard]</a>, end of class template <code>lock_guard</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
<del>template&lt;class Mutex&gt; lock_guard(lock_guard&lt;Mutex&gt;) -&gt; lock_guard&lt;Mutex&gt;;</del>
</pre>
</blockquote>
</li>

<li><p>Edit 32.6.5.3 <a href="https://wg21.link/thread.lock.scoped">[thread.lock.scoped]</a>, end of class template <code>scoped_lock</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
<del>template&lt;class... MutexTypes&gt;
  scoped_lock(scoped_lock&lt;MutexTypes...&gt;) -&gt; scoped_lock&lt;MutexTypes...&gt;;</del>
</pre>
</blockquote>
</li>

<li><p>Edit 32.6.5.4 <a href="https://wg21.link/thread.lock.unique">[thread.lock.unique]</a>, end of class template <code>unique_lock</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
<del>template&lt;class Mutex&gt; unique_lock(unique_lock&lt;Mutex&gt;) -&gt; unique_lock&lt;Mutex&gt;;</del>
</pre>
</blockquote>
</li>

<li><p>Edit 32.6.5.5 <a href="https://wg21.link/thread.lock.shared">[thread.lock.shared]</a>, end of class template <code>shared_lock</code> synopsis, 
as indicated:</p>

<blockquote>
<pre>
<del>template&lt;class Mutex&gt; shared_lock(shared_lock&lt;Mutex&gt;) -&gt; shared_lock&lt;Mutex&gt;;</del>
</pre>
</blockquote>
</li>
</ol>





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