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New issue from Hewill: "optional::value_or return statement is inconsistent with Mandates"
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‎xml/issue4406.xml‎

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<?xml version='1.0' encoding='utf-8' standalone='no'?>
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<!DOCTYPE issue SYSTEM "lwg-issue.dtd">
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<issue num="4406" status="New">
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<title>`optional::value_or` return statement is inconsistent with <i>Mandates</i></title>
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<section>
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<sref ref="[optional.observe]"/>
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<sref ref="[optional.ref.observe]"/>
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<sref ref="[expected.object.obs]"/>
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</section>
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<submitter>Hewill Kang</submitter>
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<date>06 Sep 2025</date>
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<priority>99</priority>
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<discussion>
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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 `T` can be convert from `U` when `optional` has no value.
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<p/>
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However, the return statement explicitly constructs `T` by `static_cast`, which is not checked by
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`is_convertible_v` 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 `value_or` 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 `expected::value_or`.
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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 <iref ref="4281"/>.
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</p>
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</discussion>
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<resolution>
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<p>
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This wording is relative to <paper num="N5014"/>.
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</p>
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<ol>
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<li><p>Modify <sref ref="[optional.observe]"/> 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 `true`.
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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 `true`.
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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 <sref ref="[optional.ref.observe]"/> 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 `true`.
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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 <sref ref="[expected.object.obs]"/> 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 `true` and
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<code>is_convertible_v&lt;U, T&gt;</code> is `true`.
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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 `true` and
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<code>is_convertible_v&lt;U, T&gt;</code> is `true`.
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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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</ol>
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</resolution>
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</issue>

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