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<title>Issue 2934: optional&lt;const T&gt; doesn't compare with T</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="2934"><a href="lwg-defects.html#2934">2934</a>. <code>optional&lt;const T&gt;</code> doesn't compare with <code>T</code></h3>
<p><b>Section:</b> 22.5.9 <a href="https://wg21.link/optional.comp.with.t">[optional.comp.with.t]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Ville Voutilainen <b>Opened:</b> 2017-02-17 <b>Last modified:</b> 2021-06-06</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#optional.comp.with.t">issues</a> in [optional.comp.with.t].</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>
Consider:
</p>
<blockquote><pre>
optional&lt;const int&gt; x = 42;
int y = 666;
x == y; // ill-formed
</pre></blockquote>
<p>
The comparison is ill-formed, because in [optional.comp_with_t]/1,
</p>
<pre>
template &lt;class T&gt; constexpr bool operator==(const optional&lt;T&gt;&amp; x, const T&amp; v);
</pre>
<p>
the <code>T</code> is deduced to be both <code>const int</code> and <code>int</code>, which is ill-formed.
</p>
<p>
Since it became apparent that the root cause of this issue is also causing difficulties with e.g.
comparing <code>optional&lt;string&gt;</code> with literals etc., here's a p/r that
</p>
<ol>
<li><p>adds requirements for <code>optional</code>'s comparisons with <code>T</code></p></li>
<li><p>turns <code>optional</code>-<code>optional</code> comparisons into two-template-parameter
templates, allowing comparing mixed optionals</p></li>
<li><p>turns <code>optional</code>-<code>T</code> comparisons into two-template-parameter templates,
allowing comparing optionals with <code>T</code> and types comparable with <code>T</code></p></li>
</ol>

<p><i>[Kona 2017-02-28]</i></p>

<p>Accepted as Immediate to avoid having break ABI later.</p>


<p id="res-2934"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4640">N4640</a>.</p>

<ol>
<li><p>Modify 22.5.2 <a href="https://wg21.link/optional.syn">[optional.syn]</a>, header <code>&lt;optional&gt;</code> synopsis, as indicated:</p>
<blockquote>
<pre>
[&hellip;]

<i>// 22.5.7 <a href="https://wg21.link/optional.relops">[optional.relops]</a>, relational operators</i>
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator==(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator!=(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator&lt;(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator&gt;(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator&lt;=(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt;
constexpr bool operator&gt;=(const optional&lt;T&gt;&amp;, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp;);

[&hellip;]

<i>//  [optional.comp_with_t], comparison with T</i>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator==(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator==(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator!=(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator!=(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;=(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;=(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;=(const optional&lt;T&gt;&amp;, const <del>T</del><ins>U</ins>&amp;);
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;=(const <del>T</del><ins>U</ins>&amp;, const optional&lt;T&gt;&amp;);
</pre>
</blockquote>
</li>

<li><p>Modify 22.5.7 <a href="https://wg21.link/optional.relops">[optional.relops]</a> as indicated:</p>
<blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator==(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator!=(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;=(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;=(const optional&lt;T&gt;&amp; x, const optional&lt;<del>T</del><ins>U</ins>&gt;&amp; y);
</pre>
<blockquote>
<p>
[&hellip;]
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify  [optional.comp_with_t] as indicated:</p>
<blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator==(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x == v</code> shall be well-formed and its result shall be convertible to <code>bool</code>.
 [<i>Note:</i> <code>T</code> need not be <code>EqualityComparable</code>. &mdash; <i>end note</i>]</ins>
<p/>
-1- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x == v : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator==(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v == *x</code> shall be well-formed and its result shall be convertible to <code>bool</code>.</ins>
<p/>
-2- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v == *x : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator!=(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x != v</code> shall be well-formed and its result shall be convertible to <code>bool</code>.</ins>
<p/>
-3- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x != v : true;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator!=(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v != *x</code> shall be well-formed and its result shall be convertible to <code>bool</code>.</ins>
<p/>
-4- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v != *x : true;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x &lt; v</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-5- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x &lt; v : true;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v &lt; *x</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-6- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v &lt; *x : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;=(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x &lt;= v</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-7- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x &lt;= v : true;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&lt;=(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v &lt;= *x</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-8- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v &lt;= *x : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x &gt; v</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-9- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x &gt; v : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v &gt; *x</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-10- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v &gt; *x : true;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;=(const optional&lt;T&gt;&amp; x, const <del>T</del><ins>U</ins>&amp; v);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>*x &gt;= v</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-11- <i>Effects:</i> Equivalent to: <code>return bool(x) ? *x &gt;= v : false;</code>
</p>
</blockquote>
<pre>
template &lt;class T<ins>, class U</ins>&gt; constexpr bool operator&gt;=(const <del>T</del><ins>U</ins>&amp; v, const optional&lt;T&gt;&amp; x);
</pre>
<blockquote>
<p>
<ins>-?- <i>Requires:</i> The expression <code>v &gt;= *x</code> shall be well-formed and its result
 shall be convertible to <code>bool</code>.</ins>
<p/>
-12- <i>Effects:</i> Equivalent to: <code>return bool(x) ? v &gt;= *x : true;</code>
</p>
</blockquote>
<pre>

</pre>
</blockquote>
</li>
</ol>





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