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<title>Issue 1315: return type of async</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#NAD_Editorial">NAD Editorial</a> status.</em></p>
<h3 id="1315"><a href="lwg-closed.html#1315">1315</a>. return type of <code>async</code></h3>
<p><b>Section:</b> 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> <b>Status:</b> <a href="lwg-active.html#NAD_Editorial">NAD Editorial</a>
 <b>Submitter:</b> Jonathan Wakely <b>Opened:</b> 2009-02-09 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#futures.async">active issues</a> in [futures.async].</p>
<p><b>View all other</b> <a href="lwg-index.html#futures.async">issues</a> in [futures.async].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#NAD Editorial">NAD Editorial</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Both overloads of <code>async</code> return <code>future&lt;typename
F::result_type&gt;</code> which requires that <code>F</code> has a nested type. This
prevents <code>async</code> being used with function pointers and makes the example
in 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> invalid. I believe this is unintentional.
</p>

<p>
The proposed resolution also addresses editorial issues with the
<code>launch_policy</code> function parameter.
</p>

<p>
For the first overload it is not sufficient to return <code>future&lt;typename
result_of&lt;F(ArgTypes...)&gt;::type&gt;</code>.  Calling <code>async(launch::xxx,
foo, bar)</code> performs argument deduction on both <code>async</code> overloads,
which for the first overload attempts to instantiate <code>result_of&lt;launch(F,
ArgTypes...)&gt;</code>, which is invalid. SFINAE must be used to prevent that.
</p>

<p><i>[
2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>


<p><i>[
2010-02-12 Daniel opens:
]</i></p>


<blockquote>
<p>
[..] if <code>decay&lt;F&gt;::type</code> is of type <code>std::launch</code>.
</p>
<p>
or
</p>
<p>
[..] if <code>remove_cv&lt;remove_reference&lt;F&gt;::type&gt;::type</code> is of
type <code>std::launch</code>.
</p>

<p>
The latter is the more specific form, but the former is equivalent to
the latter for all cases that can occur here. I suggest to use the
former for simplicity, but expect that implementations can effectively
use the latter.

</p>
</blockquote>

<p><i>[
2010-02-12 Moved to Tentatively Ready after 5 positive votes on c++std-lib.
]</i></p>


<p><i>[
2010 Pittsburgh:
]</i></p>


<blockquote><p>
Moved to NAD Editorial.  Rationale added below.
</p></blockquote>



<p><b>Rationale:</b></p>
<p>
Solved by N3058.
</p>


<p id="res-1315"><b>Proposed resolution:</b></p>
<p>
In 32.10.1 <a href="https://wg21.link/futures.overview">[futures.overview]</a> paragraph 1:
</p>

<blockquote><pre>
template &lt;class F, class... Args&gt;
  <del>future&lt;typename F::result_type&gt;</del>
  <ins>future&lt;typename result_of&lt;F(Args...)&gt;::type&gt;</ins>
  async(F&amp;&amp; f, Args&amp;&amp;... args);
template &lt;class F, class... Args&gt;
  <del>future&lt;typename F::result_type&gt;</del>
  <ins>future&lt;typename result_of&lt;F(Args...)&gt;::type&gt;</ins>
  async(launch policy, F&amp;&amp; f, Args&amp;&amp;... args);
</pre></blockquote>

<p>
In 32.10.9 <a href="https://wg21.link/futures.async">[futures.async]</a> before paragraph 1
</p>

<blockquote><pre>
template &lt;class F, class... Args&gt;
  <del>future&lt;typename F::result_type&gt;</del>
  <ins>future&lt;typename result_of&lt;F(Args...)&gt;::type&gt;</ins>
  async(F&amp;&amp; f, Args&amp;&amp;... args);
template &lt;class F, class... Args&gt;
  <del>future&lt;typename F::result_type&gt;</del>
  <ins>future&lt;typename result_of&lt;F(Args...)&gt;::type&gt;</ins>
  async(launch policy, F&amp;&amp; f, Args&amp;&amp;... args);
</pre>
<blockquote>
<p>...</p>
<p><ins>
<i>Remarks:</i> The first signature shall not participate in overload resolution
if <code>decay&lt;F&gt;::type</code> is <code>std::launch</code>.
</ins></p>
</blockquote>
</blockquote>






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