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<title>Issue 4299: Missing Mandates: part in optional&lt;T&amp;&gt;::transform</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="4299"><a href="lwg-active.html#4299">4299</a>. Missing <i>Mandates:</i> part in <code>optional&lt;T&amp;&gt;::transform</code></h3>
<p><b>Section:</b> 22.5.4.7 <a href="https://wg21.link/optional.ref.monadic">[optional.ref.monadic]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Giuseppe D'Angelo <b>Opened:</b> 2025-07-15 <b>Last modified:</b> 2025-08-16</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#optional.ref.monadic">active issues</a> in [optional.ref.monadic].</p>
<p><b>View all other</b> <a href="lwg-index.html#optional.ref.monadic">issues</a> in [optional.ref.monadic].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
In 22.5.4.7 <a href="https://wg21.link/optional.ref.monadic">[optional.ref.monadic]</a> the specification of <code>optional&lt;T&amp;&gt;::transform</code>
is missing an additional part of the <i>Mandates:</i> element compared to the primary template's
<code class='backtick'>transform</code> (in 22.5.3.8 <a href="https://wg21.link/optional.monadic">[optional.monadic]</a> p8); that is, is missing to enforce that
the <code class='backtick'>U</code> type is a valid contained type for <code class='backtick'>optional</code>.
<p/>
The definition of "valid contained type" comes from <a href="https://wg21.link/P2988R12" title=" std::optional<T&amp;>">P2988R12</a>. The paper
amended the <i>Mandates:</i> element of the primary template's <code class='backtick'>transform</code> to
use this definition. The fact that the same wording has not been applied to 
<code>optional&lt;T&amp;&gt;::transform</code> as well looks like an oversight. I would
suggest to apply it.
</p>


<p id="res-4299"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<ol>

<li><p>Modify 22.5.4.7 <a href="https://wg21.link/optional.ref.monadic">[optional.ref.monadic]</a> as indicated:</p>

<blockquote>
<pre>
template&lt;class F&gt;
  constexpr optional&lt;remove_cv_t&lt;invoke_result_t&lt;F, T&amp;&gt;&gt;&gt; transform(F&amp;&amp; f) const;
</pre>
<blockquote>
<p>
-4- Let <code class='backtick'>U</code> be <code>remove_cv_t&lt;invoke_result_t&lt;F, T&amp;&gt;&gt;</code>.
<p/>
-5- <i>Mandates</i>: <ins><code class='backtick'>U</code> is a valid contained type for <code class='backtick'>optional</code>.</ins> The declaration
</p>
<blockquote><pre>
U u(invoke(std::forward&lt;F&gt;(f), *val ));
</pre></blockquote>
<p>
is well-formed for some invented variable <code class='backtick'>u</code>.
</p>
</blockquote>
</blockquote>
</li>

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