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<title>Issue 2894: The function template std::apply() is required to be constexpr, but std::invoke() isn't</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#Resolved">Resolved</a> status.</em></p>
<h3 id="2894"><a href="lwg-defects.html#2894">2894</a>. The function template <code>std::apply()</code> is required to be <code>constexpr</code>, but <code>std::invoke()</code> isn't</h3>
<p><b>Section:</b> 22.10.5 <a href="https://wg21.link/func.invoke">[func.invoke]</a> <b>Status:</b> <a href="lwg-active.html#Resolved">Resolved</a>
 <b>Submitter:</b> Great Britain <b>Opened:</b> 2017-02-03 <b>Last modified:</b> 2020-09-06</p>
<p><b>Priority: </b>3
</p>
<p><b>View all other</b> <a href="lwg-index.html#func.invoke">issues</a> in [func.invoke].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#Resolved">Resolved</a> status.</p>
<p><b>Discussion:</b></p>
<b>Addresses GB 51</b>

<p>The function template <code>std::apply()</code> in 22.4.6 <a href="https://wg21.link/tuple.apply">[tuple.apply]</a> is required to be <code>constexpr</code>, 
but <code>std::invoke()</code> in 22.10.5 <a href="https://wg21.link/func.invoke">[func.invoke]</a> isn't. The most sensible implementation of <code>apply_impl()</code> 
is exactly equivalent to <code>std::invoke()</code>, so this requires implementations to have a <code>constexpr</code> version of 
<code>invoke()</code> for internal use, and the public API <code>std::invoke</code>, which must not be <code>constexpr</code> even 
though it is probably implemented in terms of the internal version.</p>
<p>Proposed change: Add <code>constexpr</code> to <code>std::invoke</code>.</p>

<p><i>[2017-02-20, Marshall adds wording]</i></p>


<p><i>[Kona 2017-03-01]</i></p>

<p>We think this needs <a href="https://wg21.link/cwg1581">CWG 1581</a> to work; accepted as Immediate to resolve NB comment.</p>
<p>Friday: CWG 1581 was not moved in Kona. Status back to Open.</p>

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

<p>Priority 3</p>

<p><i>[2020-01 Resolved by the adoption of <a href="https://wg21.link/P1065">P1065</a> in Cologne.]</i></p>



<p id="res-2894"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Modify 22.10.5 <a href="https://wg21.link/func.invoke">[func.invoke]</a> as indicated:</p>
<blockquote>
<pre>
template &lt;class F, class... Args&gt;
     <ins>constexpr</ins> result_of_t&lt;F&amp;&amp;(Args&amp;&amp;...)&gt; invoke(F&amp;&amp; f, Args&amp;&amp;... args);
</pre>
</blockquote>
</li>

</ol>





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