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<title>Issue 3877: Incorrect constraints on const-qualified monadic overloads for std::expected</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="3877"><a href="lwg-defects.html#3877">3877</a>. Incorrect constraints on <code>const</code>-qualified monadic overloads for <code>std::expected</code></h3>
<p><b>Section:</b> 22.8.6.7 <a href="https://wg21.link/expected.object.monadic">[expected.object.monadic]</a>, 22.8.7.7 <a href="https://wg21.link/expected.void.monadic">[expected.void.monadic]</a> <b>Status:</b> <a href="lwg-active.html#C++23">C++23</a>
 <b>Submitter:</b> Sy Brand <b>Opened:</b> 2023-02-09 <b>Last modified:</b> 2023-11-22</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#expected.object.monadic">issues</a> in [expected.object.monadic].</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>
The constraints for <code>and_then</code>, <code>transform</code>, <code>transform_error</code>, and <code>or_else</code> 
for <code>std::expected</code> seem incorrect for <code>const</code> overloads. E.g., from 22.8.6.7 <a href="https://wg21.link/expected.object.monadic">[expected.object.monadic]</a>
</p>
<blockquote><pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;&amp;;
[&hellip;]
</pre>
<blockquote>
<p>
<i>Constraints</i>: <code>is_move_constructible_v&lt;E&gt;</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
<p>
That constraint should likely be <code>is_move_constructible_v&lt;const E&gt;</code> for the <code>const</code>-qualified version. 
Same for the lvalue overloads, and for the three other functions, including in the <code>void</code> partial specialization. 
For example, currently this code would result in a hard compiler error inside the body of <code>transform</code> rather than 
failing the constraint:
</p>
<blockquote><pre>
const std::expected&lt;int, std::unique_ptr&lt;int&gt;&gt; e;
std::move(e).transform([](auto) { return 42; });
</pre></blockquote>
<p><strong>Previous resolution [SUPERSEDED]:</strong></p>
<blockquote class="note">
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 22.8.6.7 <a href="https://wg21.link/expected.object.monadic">[expected.object.monadic]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-2- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-6- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-10- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>T</del><ins>decltype((value()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-14- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>T</del><ins>decltype((value()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-18- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-22- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-26- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>T</del><ins>decltype((value()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-30- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>T</del><ins>decltype((value()))</ins>&gt;</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 22.8.7.7 <a href="https://wg21.link/expected.void.monadic">[expected.void.monadic]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-2- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-6- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-16- <i>Constraints</i>: <code>is_copy_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-20- <i>Constraints</i>: <code>is_move_constructible_v&lt;<del>E</del><ins>decltype((error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>

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

</blockquote>

<p><i>[Issaquah 2023-02-09; Jonathan provides improved wording]</i></p>

<p><i>[Issaquah 2023-02-09; LWG]</i></p>

<p>Move to Immediate for C++23</p>

<p><i>[2023-02-13 Approved at February 2023 meeting in Issaquah. Status changed: Immediate &rarr; WP.]</i></p>



<p id="res-3877"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N4928" title=" Working Draft, Standard for Programming Language C++">N4928</a>.
</p>

<ol>
<li><p>Modify 22.8.6.7 <a href="https://wg21.link/expected.object.monadic">[expected.object.monadic]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-2- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(error())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-6- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(std::move(error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-10- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;T<ins>, decltype(value())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto or_else(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-14- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;T<ins>, decltype(std::move(value()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-18- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(error())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-22- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(std::move(error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-26- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;T<ins>, decltype(value())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform_error(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-30- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;T<ins>, decltype(std::move(value()))</ins>&gt;</code> is <code>true</code>.
</p>
</blockquote>
</blockquote>
</li>

<li><p>Modify 22.8.7.7 <a href="https://wg21.link/expected.void.monadic">[expected.void.monadic]</a> as indicated:</p>
<blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-2- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(error())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto and_then(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-6- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(std::move(error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-16- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(error())</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>
<pre>
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) &amp;&amp;;
template&lt;class F&gt; constexpr auto transform(F&amp;&amp; f) const &amp;&amp;;
</pre>
<blockquote>
<p>
[&hellip;]
<p/>
-20- <i>Constraints</i>: <code>is_<del>copy_</del>constructible_v&lt;E<ins>, decltype(std::move(error()))</ins>&gt;</code> is <code>true</code>.
<p/>
[&hellip;]
</p>
</blockquote>

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






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