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<title>Issue 3424: optional::value_or should never return a cv-qualified type</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#New">New</a> status.</em></p>
<h3 id="3424"><a href="lwg-active.html#3424">3424</a>. <code>optional::value_or</code> should never return a <i>cv</i>-qualified type</h3>
<p><b>Section:</b> 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.observe]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Casey Carter <b>Opened:</b> 2020-04-02 <b>Last modified:</b> 2023-02-10</p>
<p><b>Priority: </b>3
</p>
<p><b>View other</b> <a href="lwg-index-open.html#optional.observe">active issues</a> in [optional.observe].</p>
<p><b>View all other</b> <a href="lwg-index.html#optional.observe">issues</a> in [optional.observe].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The <code>optional&lt;T&gt;::value_or</code> overloads are specified to return <code>T</code>. This seems silly when
<code>T</code> is <code>const</code> or <code>volatile</code> qualified &mdash; return types should never be <i>cv</i>-qualified.
(In the <code>volatile</code> case, it is even deprecated since merging <a href="https://wg21.link/p1152r4">P1152R4</a>
"Deprecating volatile" into the working draft.) We should strip <i>cv</i>-qualifiers from these return types.
</p>

<p><i>[2020-04-18 Issue Prioritization]</i></p>

<p>Priority to 3 after reflector discussion.</p>

<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/n4861">N4861</a>.
</p>

<ol>
<li><p>Modify 22.5.3 <a href="https://wg21.link/optional.optional">[optional.optional]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) const&amp;;
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) &amp;&amp;;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.observe]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) const&amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-17- <i>Mandates:</i> <code><del>is_copy_constructible_v&lt;T&gt;</del><ins>is_convertible_v&lt;const T&amp;, R&gt;</ins>
&amp;&amp; is_convertible_v&lt;U<del>&amp;&amp;</del>, T&gt;</code> is <code>true</code>.
<p/>
-18- <i>Effects:</i> Equivalent to:
<blockquote><pre>
return bool(*this) ? **this : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v));
</pre></blockquote>
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) &amp;&amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-19- <i>Mandates:</i> <code><del>is_move_constructible_v&lt;T&gt;</del><ins>is_convertible_v&lt;T, R&gt;</ins>
&amp;&amp; is_convertible_v&lt;U<del>&amp;&amp;</del>, T&gt;</code> is <code>true</code>.
<p/>
-20- <i>Effects:</i> Equivalent to:
<blockquote><pre>
return bool(*this) ? std::move(**this) : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v));
</pre></blockquote>
</p>
</blockquote>
</blockquote>
</li>
</ol>
</blockquote>

<p><i>[2023-02-09 Casey improves wording and expands to cover <code>expected::value_or</code>]</i></p>

<p>Since <code>expected</code> was modeled on <code>optional</code>, it has the same issue.</p>


<p id="res-3424"><b>Proposed resolution:</b></p>
<p>This wording is relative to <a href="https://wg21.link/n4928">N4928</a>.</p>

<ol>
<li><p>Modify 22.5.3.1 <a href="https://wg21.link/optional.optional.general">[optional.optional.general]</a> as indicated:</p>

<blockquote>
<pre>
[&hellip;]
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) const&amp;;
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) &amp;&amp;;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.observe]</a> as indicated:</p>

<blockquote class="note">
<p>
[<i>Drafting note:</i> The two removals of the <code>&amp;&amp;</code> in
<code>is_convertible_v&lt;U&amp;&amp;, T&gt;</code> below is a simplification to restore consistency
with the wording for <code>expected::value_or</code>.]
</p>
</blockquote>

<blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) const &amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-15- <i>Mandates:</i> <code>
<del>is_copy_constructible_v&lt;T&gt;</del> <ins>is_convertible_v&lt;const T&amp;, R&gt;</ins> &amp;&amp;
is_convertible_v&lt;U<del>&amp;&amp;</del>, <del>T</del><ins>R</ins>&gt;
</code> is <code>true</code>.
<p/>
-16- <i>Effects:</i> Equivalent to:
<blockquote><pre>
return bool(*this) ? **this : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v));
</pre></blockquote>
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) &amp;&amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-17- <i>Mandates:</i> <code>
<del>is_move_constructible_v&lt;T&gt;</del> <ins>is_convertible_v&lt;T, R&gt;</ins> &amp;&amp;
is_convertible_v&lt;U<del>&amp;&amp;</del>, <del>T</del><ins>R</ins>&gt;
</code> is <code>true</code>.
<p/>
-18- <i>Effects:</i> Equivalent to:
<blockquote><pre>
return bool(*this) ? std::move(**this) : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v));
</pre></blockquote>
</p>
</blockquote>
</blockquote>
</li>

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

<blockquote>
<pre>
[&hellip;]
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) const &amp;;
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp;) &amp;&amp;;
[&hellip;]
</pre>
</blockquote>
</li>

<li><p>Modify 22.8.6.6 <a href="https://wg21.link/expected.object.obs">[expected.object.obs]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) const &amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-16- <i>Mandates:</i> <code>
<del>is_copy_constructible_v&lt;T&gt;</del> <ins>is_convertible_v&lt;const T&amp;, R&gt;</ins> &amp;&amp;
is_convertible_v&lt;U, <del>T</del><ins>R</ins>&gt;
</code> is <code>true</code>.
<p/>
-17- <i>Returns:</i>
<code>has_value() ? **this : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v))</code>.
</p>
</blockquote>
<pre>
template&lt;class U&gt; constexpr <ins>remove_cv_t&lt;</ins>T<ins>&gt;</ins> value_or(U&amp;&amp; v) &amp;&amp;;
</pre>
<blockquote>
<p>
<ins>-?- Let <code>R</code> be <code>remove_cv_t&lt;T&gt;</code>.</ins>
<p/>
-18- <i>Mandates:</i> <code>
<del>is_move_constructible_v&lt;T&gt;</del> <ins>is_convertible_v&lt;T, R&gt;</ins> &amp;&amp;
is_convertible_v&lt;U, <del>T</del><ins>R</ins>&gt;
</code> is <code>true</code>.
<p/>
-19- <i>Returns:</i>
<code>has_value() ? std::move(**this) : static_cast&lt;<del>T</del><ins>R</ins>&gt;(std::forward&lt;U&gt;(v))</code>.
</p>
</blockquote>
</blockquote>
</li>
</ol>




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