<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Issue 4304: std::optional&lt;NonReturnable&amp;&gt; is ill-formed due to value_or</title>
<meta property="og:title" content="Issue 4304: std::optional&lt;NonReturnable&amp;&gt; is ill-formed due to value_or">
<meta property="og:description" content="C++ library issue. Status: New">
<meta property="og:url" content="https://cplusplus.github.io/LWG/issue4304.html">
<meta property="og:type" content="website">
<meta property="og:image" content="http://cplusplus.github.io/LWG/images/cpp_logo.png">
<meta property="og:image:alt" content="C++ logo">
<style>
  p {text-align:justify}
  li {text-align:justify}
  pre code.backtick::before { content: "`" }
  pre code.backtick::after { content: "`" }
  blockquote.note
  {
    background-color:#E0E0E0;
    padding-left: 15px;
    padding-right: 15px;
    padding-top: 1px;
    padding-bottom: 1px;
  }
  ins {background-color:#A0FFA0}
  del {background-color:#FFA0A0}
  table.issues-index { border: 1px solid; border-collapse: collapse; }
  table.issues-index th { text-align: center; padding: 4px; border: 1px solid; }
  table.issues-index td { padding: 4px; border: 1px solid; }
  table.issues-index td:nth-child(1) { text-align: right; }
  table.issues-index td:nth-child(2) { text-align: left; }
  table.issues-index td:nth-child(3) { text-align: left; }
  table.issues-index td:nth-child(4) { text-align: left; }
  table.issues-index td:nth-child(5) { text-align: center; }
  table.issues-index td:nth-child(6) { text-align: center; }
  table.issues-index td:nth-child(7) { text-align: left; }
  table.issues-index td:nth-child(5) span.no-pr { color: red; }
  @media (prefers-color-scheme: dark) {
     html {
        color: #ddd;
        background-color: black;
     }
     ins {
        background-color: #225522
     }
     del {
        background-color: #662222
     }
     a {
        color: #6af
     }
     a:visited {
        color: #6af
     }
     blockquote.note
     {
        background-color: rgba(255, 255, 255, .10)
     }
  }
</style>
</head>
<body>
<hr>
<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="4304"><a href="lwg-active.html#4304">4304</a>. <code>std::optional&lt;<i>NonReturnable</i>&amp;&gt;</code> is ill-formed due to <code class='backtick'>value_or</code></h3>
<p><b>Section:</b> 22.5.4.6 <a href="https://wg21.link/optional.ref.observe">[optional.ref.observe]</a> <b>Status:</b> <a href="lwg-active.html#New">New</a>
 <b>Submitter:</b> Jiang An <b>Opened:</b> 2025-07-25 <b>Last modified:</b> 2025-08-16</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all issues with</b> <a href="lwg-status.html#New">New</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Currently, if <code class='backtick'>T</code> is an array type or a function type, instantiation of <code>std::optional&lt;T&amp;&gt;</code> 
is still ill-formed, because the return type of its <code class='backtick'>value_or</code> member function is specified as 
<code>remove_cv_t&lt;T&gt;</code>, which is invalid as a return type.
<p/>
However, we don't exclude such <code>T&amp;</code> from valid contained types. Given only <code class='backtick'>value_or</code> is 
problematic here, perhaps we can avoid providing it if <code class='backtick'>T</code> is not returnable.
</p>


<p id="res-4304"><b>Proposed resolution:</b></p>
<p>
This wording is relative to <a href="https://wg21.link/N5014">N5014</a>.
</p>

<ol>

<li><p>Modify 22.5.4.1 <a href="https://wg21.link/optional.optional.ref.general">[optional.optional.ref.general]</a>, header <code>&lt;iterator&gt;</code> synopsis, as indicated:</p>

<blockquote>
<pre>
namespace std {
  template&lt;class T&gt;
  class optional&lt;T&amp;&gt; {
    [&hellip;]
    constexpr T&amp; value() const; // freestanding-deleted
    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; <ins>// <i>not always present</i></ins>
    [&hellip;]
  };
}
</pre>
</blockquote>
</li>

<li><p>Modify 22.5.4.6 <a href="https://wg21.link/optional.ref.observe">[optional.ref.observe]</a> as indicated:</p>

<blockquote>
<pre>
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;
</pre>
<blockquote>
<p>
-8- Let <code class='backtick'>X</code> be <code>remove_cv_t&lt;T&gt;</code>.
<p/>
-9- <i>Mandates</i>: <code>is_constructible_v&lt;X, T&amp;&gt; &amp;&amp; is_convertible_v&lt;U, X&gt;</code> is <code class='backtick'>true</code>.
<p/>
-10- <i>Effects</i>: Equivalent to:
</p>
<blockquote><pre>
return has_value() ? *<i>val</i> : static_cast&lt;X&gt;(std::forward&lt;U&gt;(u));
</pre></blockquote>
<p>
<ins>-?- <i>Remarks</i>: This function template is present if and only if <code class='backtick'>T</code> is a non-array object type.</ins>
</p>
</blockquote>
</blockquote>
</li>
</ol>





</body>
</html>
