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<title>Issue 1071: is_bind_expression should derive from integral_constant&lt;bool&gt;</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++11">C++11</a> status.</em></p>
<h3 id="1071"><a href="lwg-defects.html#1071">1071</a>. <code>is_bind_expression</code> should derive from <code>integral_constant&lt;bool&gt;</code></h3>
<p><b>Section:</b> 22.10.15.2 <a href="https://wg21.link/func.bind.isbind">[func.bind.isbind]</a> <b>Status:</b> <a href="lwg-active.html#C++11">C++11</a>
 <b>Submitter:</b> Alisdair Meredith <b>Opened:</b> 2009-03-19 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#func.bind.isbind">issues</a> in [func.bind.isbind].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++11">C++11</a> status.</p>
<p><b>Discussion:</b></p>

<p>
Class template is_bind_expression 22.10.15.2 <a href="https://wg21.link/func.bind.isbind">[func.bind.isbind]</a>:
</p>

<blockquote><pre>
namespace std {
  template&lt;class T&gt; struct is_bind_expression {
    static const bool value = see below;
  };
}
</pre></blockquote>
<p>
<code>is_bind_expression</code> should derive from <code>std::integral_constant&lt;bool&gt;</code> like
other similar trait types.
</p>

<p><i>[
Daniel adds:
]</i></p>

<blockquote><p>
We need the same thing for the trait <code>is_placeholder</code> as well.
</p></blockquote>
<p><i>[
2009-03-22 Daniel provided wording.
]</i></p>


<p><i>[
Batavia (2009-05):
]</i></p>

<blockquote>
<p>
We recommend this be deferred until after the next Committee Draft is issued.
</p>
<p>
Move to Open.
</p>
</blockquote>

<p><i>[
2009-05-31 Peter adds:
]</i></p>


<blockquote>
<p>
I am opposed to the proposed resolution and to the premise of the issue
in general. The traits's default definitions should NOT derive from
<code>integral_constant</code>, because this is harmful, as it misleads people into
thinking that <code>is_bind_expression&lt;E&gt;</code> always derives from
<code>integral_constant</code>, whereas it may not.
</p>
<p>
<code>is_bind_expression</code> and <code>is_placeholder</code> allow user
specializations, and in fact, this is their primary purpose. Such user
specializations may not derive from <code>integral_constant</code>, and the
places where <code>is_bind_expression</code> and <code>is_placeholder</code> are
used intentionally do not require such derivation.
</p>
<p>
The long-term approach here is to switch to
<code>BindExpression&lt;E&gt;</code> and <code>Placeholder&lt;P&gt;</code>
explicit concepts, of course, but until that happens, I say leave them
alone.
</p>
</blockquote>

<p><i>[
2009-10 post-Santa Cruz:
]</i></p>


<blockquote><p>
Move to Tentatively Ready.  We are comfortable with requiring user specializations
to derive from <code>integral_constant</code>.
</p></blockquote>



<p id="res-1071"><b>Proposed resolution:</b></p>
<ol>
<li>
<p>
In 22.10.15.2 <a href="https://wg21.link/func.bind.isbind">[func.bind.isbind]</a> change as indicated:
</p>
<blockquote><pre>
namespace std {
 template&lt;class T&gt; struct is_bind_expression <ins>: integral_constant&lt;bool, <i>see below</i>&gt; { };</ins><del>{
   static const bool value = <i>see below</i>;
 };</del>
}
</pre></blockquote>
</li>
<li>
<p>
In 22.10.15.2 <a href="https://wg21.link/func.bind.isbind">[func.bind.isbind]</a>/2 change as indicated:
</p>
<blockquote><pre>
<del>static const bool value;</del>
</pre>
<blockquote><p>
-2- <del><code>true</code> if <code>T</code> is a type returned from <code>bind</code>, <code>false</code> otherwise.</del>
  <ins>If <code>T</code> is a type returned from <code>bind</code>, <code>is_bind_expression&lt;T&gt;</code> shall
be publicly derived from
        <code>integral_constant&lt;bool, true&gt;</code>, otherwise it shall be
publicly derived from
          <code>integral_constant&lt;bool, false&gt;</code>.</ins>
</p></blockquote>
</blockquote>
</li>
<li>
<p>
In 22.10.15.3 <a href="https://wg21.link/func.bind.isplace">[func.bind.isplace]</a> change as indicated:
</p>
<blockquote><pre>
namespace std {
 template&lt;class T&gt; struct is_placeholder <ins>: integral_constant&lt;int, <i>see below</i>&gt; { };</ins><del>{
   static const int value = <i>see below</i>;
 };</del>
}
</pre></blockquote>
</li>
<li>
<p>
In 22.10.15.3 <a href="https://wg21.link/func.bind.isplace">[func.bind.isplace]</a>/2 change as indicated:
</p>
<blockquote><pre>
<del>static const int value;</del>
</pre>
<blockquote><p>
-2- <del>value is <code>J</code> if <code>T</code> is the type of <code>std::placeholders::_J</code>, 0 otherwise.</del>
  <ins>If <code>T</code> is the type of <code>std::placeholders::_J</code>, <code>is_placeholder&lt;T&gt;</code>
shall be publicly
          derived from <code>integral_constant&lt;int, J&gt;</code> otherwise it shall
be publicly derived
          from <code>integral_constant&lt;int, 0&gt;</code>.</ins>
</p></blockquote>
</blockquote>
</li>
</ol>





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