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<title>Issue 2557: Logical operator traits are broken in the zero-argument case</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++17">C++17</a> status.</em></p>
<h3 id="2557"><a href="lwg-defects.html#2557">2557</a>. Logical operator traits are broken in the zero-argument case</h3>
<p><b>Section:</b> 21.3.10 <a href="https://wg21.link/meta.logical">[meta.logical]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Geoffrey Romer <b>Opened:</b> 2015-11-05 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#meta.logical">issues</a> in [meta.logical].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The conjunction trait in 21.3.10 <a href="https://wg21.link/meta.logical">[meta.logical]</a> seems intended to support invocation with zero arguments, e.g. 
<code>conjunction&lt;&gt;::value</code>, which is likely to be a useful feature. However, the specification doesn't 
actually make sense in the zero-argument case. See 21.3.10 <a href="https://wg21.link/meta.logical">[meta.logical]</a>/p3:
</p>
<blockquote><p>
The BaseCharacteristic of a specialization <code>conjunction&lt;B1, &hellip;, BN&gt;</code> is the first type <code>B</code> in 
the list <code>true_type</code>, <code>B1</code>, &hellip;, <code>BN</code> for which <code>B::value == false</code>, or if every 
<code>B::value != false</code> the BaseCharacteristic is <code>BN</code>.
</p></blockquote>
<p>
If "<code>B1, ..., BN</code>" is an empty list, then every <code>B::value != false</code>, so the BaseCharacteristic is <code>BN</code>, 
but there is no <code>BN</code> in this case. 
<p/>
(If LWG concludes that <code>conjunction</code> intentionally requires at least one argument, I would appreciate their confirmation that 
I can editorially remove the mention of <code>true_type</code>, which seems to have no normative impact outside the zero-argument case.)
<p/>
Similar comments apply to the <code>disjunction</code> trait, and to the corresponding traits in the Fundamentals working 
paper, see LWG <a href="lwg-defects.html#2558" title="[fund.ts.v2] Logical operator traits are broken in the zero-argument case (Status: TS)">2558</a><sup><a href="https://cplusplus.github.io/LWG/issue2558" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2016-02, Issues Telecon]</i></p>

<p>
P0; move to Tentatively Ready.
</p>


<p id="res-2557"><b>Proposed resolution:</b></p>
<p>This wording is relative to N4567.</p>

<ol>
<li><p>Revise 21.3.10 <a href="https://wg21.link/meta.logical">[meta.logical]</a> as follows:</p>

<blockquote>
<pre>
template&lt;class... B&gt; struct conjunction : <i>see below</i> { };
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-3- The BaseCharacteristic of a specialization <code>conjunction&lt;B1, ..., BN&gt;</code> is the first type <code>Bi</code> in the
list <code>true_type, B1, ..., BN</code> for which <code>Bi::value == false</code>, or if every <code>Bi::value != false</code>, the
BaseCharacteristic is <del><code>BN</code></del><ins>the last type in the list</ins>. [<i>Note</i>: This means a specialization 
of <code>conjunction</code> does not necessarily have a BaseCharacteristic of either <code>true_type</code> or <code>false_type</code>. 
&mdash; <i>end note</i>]
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class... B&gt; struct disjunction : <i>see below</i> { };
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-6- The BaseCharacteristic of a specialization <code>disjunction&lt;B1, ..., BN&gt;</code> is the first type <code>Bi</code> in the
list <code>false_type, B1, ..., BN</code> for which <code>Bi::value != false</code>, or if every <code>Bi::value == false</code>, the
BaseCharacteristic is <del><code>BN</code></del><ins>the last type in the list</ins>. [<i>Note</i>: This means a specialization 
of <code>disjunction</code> does not necessarily have a BaseCharacteristic of either <code>true_type</code> or <code>false_type</code>. 
&mdash; <i>end note</i>]
<p/>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>
</ol>





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