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<!DOCTYPE html>
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<html lang="en">
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<title>Issue 4281: Inconsistency between value_or() and error_or() in std::expected</title>
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<meta property="og:title" content="Issue 4281: Inconsistency between value_or() and error_or() in std::expected">
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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="4281"><a href="lwg-active.html#4281">4281</a>. Inconsistency between <code class='backtick'>value_or()</code> and <code class='backtick'>error_or()</code> in <code class='backtick'>std::expected</code></h3>
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<p><b>Section:</b> 22.5.3.7 <a href="https://wg21.link/optional.observe">[optional.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> Hiroaki Ando <b>Opened:</b> 2025-06-27 <b>Last modified:</b> 2025-06-27</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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In 22.8.6.6 <a href="https://wg21.link/expected.object.obs">[expected.object.obs]</a>/19, the return value of <code class='backtick'>value_or()</code>
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is specified as follows:
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<blockquote>
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<i>Returns</i>:
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<code>has_value() ? **this : static_cast&lt;T&gt;(std::forward&lt;U&gt;(v))</code>.
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</blockquote>
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Meanwhile, the return value of <code class='backtick'>error_or()</code> is specified as follows (22.8.6.6 <a href="https://wg21.link/expected.object.obs">[expected.object.obs]</a>/23):
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<blockquote>
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<i>Returns</i>:
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<code>std::forward&lt;G&gt;(e)</code> if <code>has_value()</code> is <code>true</code>, <code>error()</code> otherwise.
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</blockquote>
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Since these functions appear to be dual in nature,
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it would be preferable to maintain consistent notation.
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</p>
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<p>
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Jonathan adds:
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The wording in <code class='backtick'>expected::error_or</code> is newer, having been added by
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<a href="https://wg21.link/P2505R5" title=" Monadic Functions for std::expected">P2505R5</a>, and intentionally avoided a conditional expression
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(the problems with conditional expressions explained in <a href="https://wg21.link/P3177R0" title=" const prvalues in the conditional operator">P3177R0</a>
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don't actually affect these member functions, due to the non-const prvalue
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return type, but determining that there are no pessimized copies in <code class='backtick'>value_or</code>
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wouldn't be necessary if we didn't specify it with a conditional expression).
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The <code class='backtick'>error_or</code> wording also avoids using an explicit conversion when the
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<i>Mandates</i>: element requires implicit conversion to work anyway.
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We might want to rephrase the <code class='backtick'>value_or</code> wording to match <code class='backtick'>error_or</code>,
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or possibly make <code class='backtick'>value_or</code> and <code class='backtick'>error_or</code> even more explicit:
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</p>
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<blockquote>
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<i>Effects</i>: Equivalent to:
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<pre><code>
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if (has_value())
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return **this;
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else
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return std::forward&lt;U&gt;(v);
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</code></pre>
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</blockquote>
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<p id="res-4281"><b>Proposed resolution:</b></p>
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<p>
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</p>
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</body>
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</html>

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