New issue from Giuseppe D'Angelo: "Missing Mandates: part in optional<T&>::transform"

Author: Dani-Hub

xml/issue4299.xml

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+<?xml version='1.0' encoding='utf-8' standalone='no'?>
+<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
+
+<issue num="4299" status="New">
+<title>Missing <i>Mandates:</i> part in <tt>optional&lt;T&amp;&gt;::transform</tt></title>
+<section><sref ref="[optional.ref.monadic]"/></section>
+<submitter>Giuseppe D'Angelo</submitter>
+<date>15 Jul 2025</date>
+<priority>99</priority>
+
+<discussion>
+<p>
+In <sref ref="[optional.ref.monadic]"/> the specification of <tt>optional&lt;T&amp;&gt;::transform</tt>
+is missing an additional part of the <i>Mandates:</i> element compared to the primary template's
+`transform` (in <sref ref="[optional.monadic]"/> p8; that is, is missing to enforce that
+the `U` type is a valid contained type for `optional`.
+<p/>
+The definition of "valid contained type" comes from <paper num="P2988R12"/>. The paper
+amended the <i>Mandates:</i> element of the primary template's `transform` to
+use this definition. The fact that the same wording has not been applied to 
+<tt>optional&lt;T&amp;&gt;::transform</tt> as well looks like an oversight. I would
+suggest to apply it.
+</p>
+</discussion>
+
+<resolution>
+<p>
+This wording is relative to this 
+<a href="https://github.com/cplusplus/draft/actions/runs/16433597877/artifacts/3583518547">CD preview draft</a>.
+</p>
+
+<ol>
+
+<li><p>Modify <sref ref="[optional.ref.monadic]"/> 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 `U` be <tt>remove_cv_t&lt;invoke_result_t&lt;F, T&amp;&gt;&gt;</tt>.
+<p/>
+-5- <i>Mandates</i>: <ins>`U` is a valid contained type for `optional`.</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 `u`.
+</p>
+</blockquote>
+</blockquote>
+</li>
+
+</ol>
+</resolution>
+
+</issue>