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<title>Issue 2539: [fund.ts.v2] invocation_trait definition definition doesn't work for surrogate call functions</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#TS">TS</a> status.</em></p>
<h3 id="2539"><a href="lwg-defects.html#2539">2539</a>. [fund.ts.v2] <code>invocation_trait</code> definition definition doesn't work for surrogate call functions</h3>
<p><b>Section:</b> 3.3.2 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#meta.trans.other">[fund.ts.v2::meta.trans.other]</a> <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> Mike Spertus <b>Opened:</b> 2015-09-25 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#TS">TS</a> status.</p>
<p><b>Discussion:</b></p>
<p><b>Addresses: fund.ts.v2</b></p>
<p>
In Library Fundamentals 2 (<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4529.html">N4529</a>) 
3.3.2p3 [meta.trans.other], the definition of invocation traits for a class object <code>f</code> considers when <code>f</code> 
is called via a function call operator that is matched by the arguments but ignores the possibility that <code>f</code> 
may be called via a surrogate call function (C++14 12.2.2.2.3 <a href="https://wg21.link/over.call.object">[over.call.object]</a> p2), in which case, the definition 
of the invocation parameters may be either incorrect or even unsatisfiable.
</p>

<p><i>[2015-10, Kona Saturday afternoon]</i></p>

<p>AM: Do we have this trait yet? JW: No, it cannot be implemented without compiler support.</p>
<p>Move to tentatively ready</p>


<p id="res-2539"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4529.html">N4529</a>.</p>

<ol>
<li><p>In Library Fundamentals 2, change [meta.trans.other] as indicated:</p>
<blockquote>
<p>
-3- Within this section, define the <i>invocation parameters</i> of <code><i>INVOKE</i>(f, t1, t2, ..., tN)</code> as follows, 
in which <code>T1</code> is the possibly <i>cv</i>-qualified type of <code>t1</code> and <code>U1</code> denotes <code>T1&amp;</code> if 
<code>t1</code> is an lvalue or <code>T1&amp;&amp;</code> if <code>t1</code> is an rvalue: 
</p>
<ul>
<li><p>[&hellip;]</p></li>
<li><p>If <code>f</code> is a class object, the <i>invocation parameters</i> are the parameters matching <code>t1, ..., tN</code> 
of the best viable function (C++14 &sect;13.3.3) for the arguments <code>t1, ..., tN</code> among the function call operators 
<ins>and surrogate call functions</ins> of <code>f</code>. </p></li>
<li><p>[&hellip;]</p></li>
</ul>
</blockquote>
</li>
</ol>





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