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<title>Issue 3617: function/packaged_task deduction guides and deducing this</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="3617"><a href="lwg-defects.html#3617">3617</a>. <code>function</code>/<code>packaged_task</code> deduction guides and deducing <code>this</code></h3>
<p><b>Section:</b> 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>, 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Barry Revzin <b>Opened:</b> 2021-10-09 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>2
</p>
<p><b>View all other</b> <a href="lwg-index.html#func.wrap.func.con">issues</a> in [func.wrap.func.con].</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>
With the adoption of deducing <code>this</code> (<a href="https://wg21.link/p0847">P0847</a>), we can now create 
types whose call operator is an explicit object member function, which means that <code>decltype(&amp;F::operator())</code> 
could have pointer-to-function type rather than pointer-to-member-function type. This means that the deduction 
guides for <code>std::function</code> (22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a>) and <code>std::packaged_task</code> 
(32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a>) will simply fail:
</p>
<blockquote><pre>
struct F {
  int operator()(this const F&amp;) { return 42; }
};

std::function g = F{}; // error: decltype(&amp;F::operator()) is not of the form R(G::*)(A...) <i>cv</i> &amp;<sub>opt</sub> noexcept<sub>opt</sub>
</pre></blockquote>
<p>
We should update the deduction guides to keep them in line with the core language.
</p>

<p><i>[2021-10-14; Reflector poll]</i></p>

<p>
Set priority to 2 after reflector poll.
</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F&gt; function(F) -&gt; function&lt;<i>see below</i>&gt;;
</pre>
<blockquote>
<p>
-16- <i>Constraints:</i> <code>&amp;F::operator()</code> is well-formed when treated as an unevaluated operand and 
<code>decltype(&amp;F::operator())</code> is <ins>either</ins> of the form <code>R(G::*)(A...) <i>cv</i> 
&amp;<sub>opt</sub> noexcept<sub>opt</sub></code> <ins>or of the form <code>R(*)(G <i>cv</i> &amp;<sub>opt</sub>, A...) 
noexcept<sub>opt</sub></code></ins> for a <del>class</del> type <code>G</code>.
<p/>
-17- <i>Remarks:</i> The deduced type is <code>function&lt;R(A...)&gt;</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F&gt; packaged_task(F) -&gt; packaged_task&lt;<i>see below</i>&gt;;
</pre>
<blockquote>
<p>
-7- <i>Constraints:</i> <code>&amp;F::operator()</code> is well-formed when treated as an unevaluated operand and 
<code>decltype(&amp;F::operator())</code> is <ins>either</ins> of the form <code>R(G::*)(A...) <i>cv</i> 
&amp;<sub>opt</sub> noexcept<sub>opt</sub></code> <ins>or of the form <code>R(*)(G <i>cv</i> &amp;<sub>opt</sub>, A...) 
  noexcept<sub>opt</sub></code></ins> for a <del>class</del> type <code>G</code>.
<p/>
-8- <i>Remarks:</i> The deduced type is <code>packaged_task&lt;R(A...)&gt;</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2021-10-17; Improved wording based on Tim Song's suggestion]</i></p>


<p><i>[2022-07-01; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after nine votes in favour during reflector poll.
</p>

<p><i>[2022-07-15; LWG telecon: move to Ready]</i></p>


<p><i>[2022-07-25 Approved at July 2022 virtual plenary. Status changed: Ready &rarr; WP.]</i></p>



<p id="res-3617"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/n4892">N4892</a>.
</p>

<ol>
<li><p>Modify 22.10.17.3.2 <a href="https://wg21.link/func.wrap.func.con">[func.wrap.func.con]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F&gt; function(F) -&gt; function&lt;<i>see below</i>&gt;;
</pre>
<blockquote>
<p>
-16- <i>Constraints:</i> <code>&amp;F::operator()</code> is well-formed when treated as an unevaluated operand and 
<code>decltype(&amp;F::operator())</code> is <ins>either</ins> of the form <code>R(G::*)(A...) <i>cv</i> 
&amp;<sub>opt</sub> noexcept<sub>opt</sub></code> <ins>or of the form <code>R(*)(G, A...) noexcept<sub>opt</sub></code></ins> 
for a <del>class</del> type <code>G</code>.
<p/>
-17- <i>Remarks:</i> The deduced type is <code>function&lt;R(A...)&gt;</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 32.10.10.2 <a href="https://wg21.link/futures.task.members">[futures.task.members]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F&gt; packaged_task(F) -&gt; packaged_task&lt;<i>see below</i>&gt;;
</pre>
<blockquote>
<p>
-7- <i>Constraints:</i> <code>&amp;F::operator()</code> is well-formed when treated as an unevaluated operand and 
<code>decltype(&amp;F::operator())</code> is <ins>either</ins> of the form <code>R(G::*)(A...) <i>cv</i> 
&amp;<sub>opt</sub> noexcept<sub>opt</sub></code> <ins>or of the form <code>R(*)(G, A...) noexcept<sub>opt</sub></code></ins> 
for a <del>class</del> type <code>G</code>.
<p/>
-8- <i>Remarks:</i> The deduced type is <code>packaged_task&lt;R(A...)&gt;</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>





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