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<title>Issue 2733: [fund.ts.v2] gcd / lcm and bool</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="2733"><a href="lwg-defects.html#2733">2733</a>. [fund.ts.v2] <code>gcd</code> / <code>lcm</code> and <code>bool</code></h3>
<p><b>Section:</b> 13.1.2 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#numeric.ops.gcd">[fund.ts.v2::numeric.ops.gcd]</a>, 13.1.3 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#numeric.ops.lcm">[fund.ts.v2::numeric.ops.lcm]</a> <b>Status:</b> <a href="lwg-active.html#TS">TS</a>
 <b>Submitter:</b> Richard Smith <b>Opened:</b> 2016-06-15 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>4
</p>
<p><b>View all other</b> <a href="lwg-index.html#fund.ts.v2::numeric.ops.gcd">issues</a> in [fund.ts.v2::numeric.ops.gcd].</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>
According to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4562.html">N4562</a>, <code>gcd</code> and 
<code>lcm</code> support <code>bool</code> as the operand type. The wording doesn't appear to cover the behavior for that case, 
since <code>bool</code> does not have a zero value and <code>gcd</code> / <code>lcm</code> are not normally mathematically defined 
over <code>{false, true}</code>.
<p/>
Presumably <code>gcd</code> and <code>lcm</code> shouldn't accept arguments of type <code>bool</code>.
</p>

<p><i>[2016-08-01, Walter Brown suggests wording]</i></p>

<p>
A corresponding issue has been added addressing the WP, see LWG <a href="lwg-defects.html#2759" title="gcd / lcm and bool for the WP (Status: C++17)">2759</a><sup><a href="https://cplusplus.github.io/LWG/issue2759" title="Latest snapshot">(i)</a></sup>.
</p>

<p><i>[2016-08, Chicago]</i></p>

<p>Monday PM: Moved to Tentatively Ready</p>


<p id="res-2733"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4600.html">N4600</a>.</p>

<ol>
<li><p>Adjust 13.1.2 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#numeric.ops.gcd">[fund.ts.v2::numeric.ops.gcd]</a> p3 as indicated:</p>

<blockquote>
<pre>
template&lt;class M, class N&gt;
  constexpr common_type_t&lt;M, N&gt; gcd(M m, N n);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-3- <i>Remarks:</i> If either <code>M</code> or <code>N</code> is not an integer type, <ins>or if either is (possibly <i>cv</i>-qualified) 
<code>bool</code>,</ins> the program is ill-formed. 
</p>
</blockquote>
</blockquote>
</li>

<li><p>Adjust 13.1.3 <a href="https://cplusplus.github.io/fundamentals-ts/v2.html#numeric.ops.lcm">[fund.ts.v2::numeric.ops.lcm]</a> p3 as indicated:</p>

<blockquote>
<pre>
template&lt;class M, class N&gt;
  constexpr common_type_t&lt;M, N&gt; lcm(M m, N n);
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-3- <i>Remarks:</i> If either <code>M</code> or <code>N</code> is not an integer type, <ins>or if either is (possibly <i>cv</i>-qualified) 
<code>bool</code>,</ins> the program is ill-formed. 
</p>
</blockquote>
</blockquote>
</li>
</ol>





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