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<title>Issue 3866: Bad Mandates for expected::transform_error overloads</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="3866"><a href="lwg-defects.html#3866">3866</a>. Bad <i>Mandates</i> for <code>expected::transform_error</code> overloads</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> Casey Carter <b>Opened:</b> 2023-01-29 <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 overloads of <code>expected::transform_error</code> mandate that "[type] <code>G</code> is a valid value type for <code>expected</code>" 
(22.8.6.7 <a href="https://wg21.link/expected.object.monadic">[expected.object.monadic]</a>/27 and 31 as well as 22.8.7.7 <a href="https://wg21.link/expected.void.monadic">[expected.void.monadic]</a>/24 and 27)). 
<p/>
All of these overloads then instantiate <code>expected&lt;T, G&gt;</code> (for some type <code>T</code>) which doesn't require 
<code>G</code> to be a valid value type for <code>expected</code> (22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a>/2) but instead requires 
that <code>G</code> is "a valid template argument for <code>unexpected</code>" (22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a>/2). 
Comparing 22.8.6.1 <a href="https://wg21.link/expected.object.general">[expected.object.general]</a>/2 with 22.8.3.1 <a href="https://wg21.link/expected.un.general">[expected.un.general]</a>/2 it's clear that there are 
types &mdash; <code>const int</code>, for example &mdash; which are valid value types for <code>expected</code> but not valid 
template arguments for <code>unexpected</code>. Presumably this unimplementable requirement is a typo, and the subject 
paragraphs intended to require that <code>G</code> be a valid template argument for <code>unexpected</code>. 
</p>

<p><i>[2023-02-06; Reflector poll]</i></p>

<p>
Set status to Tentatively Ready after five votes in favour during reflector poll.
</p>

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



<p id="res-3866"><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 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/>
-27- <i>Mandates</i>: <code>G</code> is a valid <del>value type for <code>expected</code></del><ins>template argument for 
<code>unexpected</code> (22.8.3.1 <a href="https://wg21.link/expected.un.general">[expected.un.general]</a>)</ins> and the declaration
</p>
<blockquote><pre>
G g(invoke(std::forward&lt;F&gt;(f), error()));
</pre></blockquote>
<p>
is well-formed.
<p/>
[&hellip;]
</p>
</blockquote>
[&hellip;]
<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/>
-31- <i>Mandates</i>: <code>G</code> is a valid <del>value type for <code>expected</code></del><ins>template argument for 
<code>unexpected</code> (22.8.3.1 <a href="https://wg21.link/expected.un.general">[expected.un.general]</a>)</ins> and the declaration
</p>
<blockquote><pre>
G g(invoke(std::forward&lt;F&gt;(f), std::move(error())));
</pre></blockquote>
<p>
is well-formed.
<p/>
[&hellip;]
</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 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/>
-24- <i>Mandates</i>: <code>G</code> is a valid <del>value type for <code>expected</code></del><ins>template argument for 
<code>unexpected</code> (22.8.3.1 <a href="https://wg21.link/expected.un.general">[expected.un.general]</a>)</ins> and the declaration
</p>
<blockquote><pre>
G g(invoke(std::forward&lt;F&gt;(f), error()));
</pre></blockquote>
<p>
is well-formed.
<p/>
[&hellip;]
</p>
</blockquote>
[&hellip;]
<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/>
-27- <i>Mandates</i>: <code>G</code> is a valid <del>value type for <code>expected</code></del><ins>template argument for 
<code>unexpected</code> (22.8.3.1 <a href="https://wg21.link/expected.un.general">[expected.un.general]</a>)</ins> and the declaration
</p>
<blockquote><pre>
G g(invoke(std::forward&lt;F&gt;(f), std::move(error())));
</pre></blockquote>
<p>
is well-formed.
<p/>
[&hellip;]
</p>
</blockquote>

</blockquote>
</li>

</ol>




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