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<title>Issue 3754: Class template expected synopsis contains declarations that do not match the detailed description</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++23">C++23</a> status.</em></p>
<h3 id="3754"><a href="lwg-defects.html#3754">3754</a>. Class template <code>expected</code> synopsis contains declarations that do not match the detailed description</h3>
<p><b>Section:</b> 22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> S. B. Tam <b>Opened:</b> 2022-08-23 <b>Last modified:</b> 2024-01-29</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#expected.object.general">issues</a> in [expected.object.general].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++23">C++23</a> status.</p>
<p><b>Discussion:</b></p>
<p>
22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a> declares the following constructors:
</p>

<blockquote><pre>
template&lt;class G&gt;
  constexpr expected(const unexpected&lt;G&gt;&amp;);
template&lt;class G&gt;
  constexpr expected(unexpected&lt;G&gt;&amp;&amp;);
</pre></blockquote>

<p>
But in  [expected.object.ctor], these constructors are declared as:
</p>

<blockquote><pre>
template&lt;class G&gt;
  constexpr explicit(!is_convertible_v&lt;const G&amp;, E&gt;) expected(const unexpected&lt;G&gt;&amp; e);
template&lt;class G&gt;
  constexpr explicit(!is_convertible_v&lt;G, E&gt;) expected(unexpected&lt;G&gt;&amp;&amp; e);
</pre></blockquote>

<p>
Note that they have no explicit-specifiers in 22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a>,
but are conditionally explicit in  [expected.object.ctor].
</p>

<p>
I presume that 22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a>
is missing a few <code>explicit(<i>see below</i>)</code>.
</p>

<p>
The same inconsistency exists in 22.8.7 <a href="https://wg21.link/expected.void">[expected.void]</a>.
</p>

<p><i>[2022-09-05; Jonathan Wakely provides wording]</i></p>


<p>
In <a href="https://wg21.link/N4910" title=" Working Draft, Standard for Programming Language C++">N4910</a> the <code>expected</code> synopses had
<code>explicit(<i>see below</i>)</code> on the copy and move constructors.
That was fixed editorially, but this other inconsistency was not noticed.
</p>

<p><i>[2022-09-07; Moved to "Ready" at LWG telecon]</i></p>


<p><i>[2022-11-12 Approved at November 2022 meeting in Kona. Status changed: Voting &rarr; WP.]</i></p>



<p id="res-3754"><b>Proposed resolution:</b></p>

<p>
This wording is relative to <a href="https://wg21.link/N4917" title=" Working Draft, Standard for Programming Language C++">N4917</a>.
</p>

<ol>
<li><p>Change 22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a> as indicated:</p>

<blockquote><pre>
<i>// 22.8.6.2, constructors</i>
constexpr expected();
constexpr expected(const expected&amp;);
constexpr expected(expected&amp;&amp;) noexcept(<i>see below</i>);
template&lt;class U, class G&gt;
  constexpr explicit(<i>see below</i>) expected(const expected&lt;U, G&gt;&amp;);
template&lt;class U, class G&gt;
  constexpr explicit(<i>see below</i>) expected(expected&lt;U, G&gt;&amp;&amp;);

template&lt;class U = T&gt;
  constexpr explicit(<i>see below</i>) expected(U&amp;&amp; v);

template&lt;class G&gt;
  constexpr <ins>explicit(<i>see below</i>)</ins> expected(const unexpected&lt;G&gt;&amp;);
template&lt;class G&gt;
  constexpr <ins>explicit(<i>see below</i>)</ins> expected(unexpected&lt;G&gt;&amp;&amp;);

template&lt;class... Args&gt;
  constexpr explicit expected(in_place_t, Args&amp;&amp;...);
</pre> </blockquote>
</li>

<li><p>Change 22.8.7.1 <a href="https://wg21.link/expected.void.general">[expected.void.general]</a> as indicated:</p>

<blockquote><pre>
<i>// 22.8.7.2, constructors</i>
constexpr expected() noexcept;
constexpr expected(const expected&amp;);
constexpr expected(expected&amp;&amp;) noexcept(<i>see below</i>);
template&lt;class U, class G&gt;
  constexpr explicit(<i>see below</i>) expected(const expected&lt;U, G&gt;&amp;&amp;);

template&lt;class G&gt;
  constexpr <ins>explicit(<i>see below</i>)</ins> expected(const unexpected&lt;G&gt;&amp;);
template&lt;class G&gt;
  constexpr <ins>explicit(<i>see below</i>)</ins> expected(unexpected&lt;G&gt;&amp;&amp;);

constexpr explicit expected(in_place_t) noexcept;
</pre> </blockquote>

</li>

</ol>






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