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<title>Issue 2496: Certain hard-to-avoid errors not in the immediate context are not allowed to be triggered by 
the evaluation of type traits</title>
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the evaluation of type traits">
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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#New">New</a> status.</em></p>
<h3 id="2496"><a href="lwg-active.html#2496">2496</a>. Certain hard-to-avoid errors not in the immediate context are not allowed to be triggered by 
the evaluation of type traits</h3>
<p><b>Section:</b> 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Hubert Tong <b>Opened:</b> 2015-05-07 <b>Last modified:</b> 2015-08-03</p>
<p><b>Priority: </b>3
</p>
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<p><b>Discussion:</b></p>
<p>
I do not believe that the wording in 21.3.6.4 <a href="https://wg21.link/meta.unary.prop">[meta.unary.prop]</a> paragraph 3 allows for the following program to be ill-formed:
</p>
<blockquote>
<pre>
#include &lt;type_traits&gt;

template &lt;typename T&gt; struct B : T { };
template &lt;typename T&gt; struct A { A&amp; operator=(const B&lt;T&gt;&amp;); };

std::is_assignable&lt;A&lt;int&gt;, int&gt; q;
</pre>
</blockquote>
<p>
In particular, I do not see where the wording allows for the "compilation of the expression" 
<code>declval&lt;T&gt;() = declval&lt;U&gt;()</code> to occur as a consequence of instantiating <code>std::is_assignable&lt;T, U&gt;</code> 
(where <code>T</code> and <code>U</code> are, respectively, <code>A&lt;int&gt;</code> and <code>int</code> in the example code).
<p/>
Instantiating <code>A&lt;int&gt;</code> as a result of requiring it to be a complete type does not trigger the instantiation of 
<code>B&lt;int&gt;</code>; however, the "compilation of the expression" in question does.
</p>


<p id="res-2496"><b>Proposed resolution:</b></p>





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