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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8">
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<title>Issue 4406: optional::value_or return statement is inconsistent with Mandates</title>
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<meta property="og:title" content="Issue 4406: optional::value_or return statement is inconsistent with Mandates">
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<meta property="og:description" content="C++ library issue. Status: New">
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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>
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<h3 id="4406"><a href="lwg-active.html#4406">4406</a>. <code class='backtick'>optional::value_or</code> return statement is inconsistent with <i>Mandates</i></h3>
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<p><b>Section:</b> 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.observe]</a>, 22.5.4.6 <a href="https://wg21.link/optional.ref.observe">[optional.ref.observe]</a>, 22.8.6.6 <a href="https://wg21.link/expected.object.obs">[expected.object.obs]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
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<b>Submitter:</b> Hewill Kang <b>Opened:</b> 2025-09-06 <b>Last modified:</b> 2025-10-10</p>
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<p><b>Priority: </b>Not Prioritized
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</p>
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<p><b>View other</b> <a href="lwg-index-open.html#optional.observe">active issues</a> in [optional.observe].</p>
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<p><b>View all other</b> <a href="lwg-index.html#optional.observe">issues</a> in [optional.observe].</p>
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<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
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<p><b>Discussion:</b></p>
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<p>
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<code>optional&lt;T&gt;::value_or(U&amp;&amp;)</code> requires <code>is_convertible_v&lt;U&amp;&amp;, T&gt;</code>
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to ensure that <code class='backtick'>T</code> can be convert from <code class='backtick'>U</code> when <code class='backtick'>optional</code> has no value.
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<p/>
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However, the return statement explicitly constructs <code class='backtick'>T</code> by <code class='backtick'>static_cast</code>, which is not checked by
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<code class='backtick'>is_convertible_v</code> since it only checks for implicit conversions.
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<p/>
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This results in rare cases where <i>Mandates</i> may not be violated, but <code class='backtick'>value_or</code> is ill-formed
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(<a href="https://godbolt.org/z/z5jjhY7c4">demo</a>):
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</p>
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<blockquote><pre>
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struct S {
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operator int() const;
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explicit operator int() = delete;
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};
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int main() {
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std::optional&lt;int&gt;{}.value_or(S{}); // <span style="color:#C80000;font-weight:bold">fire</span>
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}
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</pre></blockquote>
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<p>
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It is reasonable to create objects that stick to <i>Mandates</i>. The same goes for <code class='backtick'>expected::value_or</code>.
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<p/>
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<b>Daniel:</b>
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<p/>
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This issue has considerable overlap with LWG <a href="lwg-active.html#4281" title="Inconsistency between value_or() and error_or() in std::expected (Status: New)">4281</a><sup><a href="https://cplusplus.github.io/LWG/issue4281" title="Latest snapshot">(i)</a></sup>.
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</p>
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<p id="res-4406"><b>Proposed resolution:</b></p>
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<p>
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This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
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</p>
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<ol>
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<li><p>Modify 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.observe]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class U = remove_cv_t&lt;T&gt;&gt; constexpr T value_or(U&amp;&amp; v) const &amp;;
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</pre>
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<blockquote>
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<p>
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-15- <i>Mandates</i>: <code>is_copy_constructible_v&lt;T&gt; &amp;&amp; is_convertible_v&lt;U&amp;&amp;, T&gt;</code>
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is <code class='backtick'>true</code>.
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<p/>
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-16- <i>Effects</i>: Equivalent to:
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</p>
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<blockquote><pre>
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<del>return has_value() ? **this : static_cast&lt;T&gt;(std::forward&lt;U&gt;(v));</del>
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<ins>if (has_value())
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return **this;
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return std::forward&lt;U&gt;(v);</ins>
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</pre></blockquote>
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</blockquote>
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<pre>
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template&lt;class U = remove_cv_t&lt;T&gt;&gt; constexpr T value_or(U&amp;&amp; v) &amp;&amp;;
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</pre>
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<blockquote>
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<p>
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-17- <i>Mandates</i>: <code>is_move_constructible_v&lt;T&gt; &amp;&amp; is_convertible_v&lt;U&amp;&amp;, T&gt;</code>
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is <code class='backtick'>true</code>.
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<p/>
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-18- <i>Effects</i>: Equivalent to:
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</p>
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<blockquote><pre>
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<del>return has_value() ? std::move(**this) : static_cast&lt;T&gt;(std::forward&lt;U&gt;(v));</del>
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<ins>if (has_value())
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return std::move(**this);
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return std::forward&lt;U&gt;(v);</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify 22.5.4.6 <a href="https://wg21.link/optional.ref.observe">[optional.ref.observe]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class U = remove_cv_t&lt;T&gt;&gt; constexpr remove_cv_t&lt;T&gt; value_or(U&amp;&amp; u) const;
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</pre>
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<blockquote>
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<p>
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-8- Let <code>X</code> be <code>remove_cv_t&lt;T&gt;</code>.
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<p/>
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-9- <i>Mandates</i>: <code>is_constructible_v&lt;X, T&amp;&gt; &amp;&amp; is_convertible_v&lt;U, X&gt;</code>
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is <code class='backtick'>true</code>.
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<p/>
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-10- <i>Effects</i>: Equivalent to:
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</p>
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<blockquote><pre>
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<del>return has_value() ? *<i>val</i> : static_cast&lt;X&gt;(std::forward&lt;U&gt;(u));</del>
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<ins>if (has_value())
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return *<i>val</i>;
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return std::forward&lt;U&gt;(u);
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</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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<li><p>Modify 22.8.6.6 <a href="https://wg21.link/expected.object.obs">[expected.object.obs]</a> as indicated:</p>
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<blockquote>
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<pre>
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template&lt;class U = remove_cv_t&lt;T&gt;&gt; constexpr T value_or(U&amp;&amp; v) const &amp;;
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</pre>
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<blockquote>
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<p>
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-18- <i>Mandates</i>: <code>is_copy_constructible_v&lt;T&gt;</code> is <code class='backtick'>true</code> and
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<code>is_convertible_v&lt;U, T&gt;</code> is <code class='backtick'>true</code>.
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<p/>
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<del>-19- <i>Returns</i>: <code>has_value() ? **this : static_cast&lt;T&gt;(std::forward&lt;U&gt;(v))</code>.</del>
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<p/>
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<ins>-?- <i>Effects</i>: Equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>if (has_value())
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return **this;
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return std::forward&lt;U&gt;(v);</ins>
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</pre></blockquote>
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</blockquote>
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<pre>
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template&lt;class U = remove_cv_t&lt;T&gt;&gt; constexpr T value_or(U&amp;&amp; v) &amp;&amp;;
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</pre>
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<blockquote>
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<p>
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-20- <i>Mandates</i>: <code>is_move_constructible_v&lt;T&gt;</code> is <code class='backtick'>true</code> and
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<code>is_convertible_v&lt;U, T&gt;</code> is <code class='backtick'>true</code>.
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<p/>
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<del>-21- <i>Returns</i>: <code>has_value() ? std::move(**this) : static_cast&lt;T&gt;(std::forward&lt;U&gt;(v))</code>.</del>
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<p/>
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<ins>-?- <i>Effects</i>: Equivalent to:</ins>
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</p>
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<blockquote><pre>
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<ins>if (has_value())
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return std::move(**this);
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return std::forward&lt;U&gt;(v);</ins>
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</pre></blockquote>
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</blockquote>
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</blockquote>
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</li>
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