<code>Y</code>. The <code>value_or_error</code> concept in turn is true if and only if the input type has:</p>

<ol>
<li>A <code>.has_value()</code> observer returning a <code>bool</code>.</li>
<li><code>.value()</code> and <code>.error()</code> observers.</li>
</ol>

<h2 id="implementation">Implementation</h2>

<p>Outcome&rsquo;s machinery for implementing <code>concepts::value_or_error</code> conversion is user extensible by injection
of specialisations of the <a href="../../../reference/converters/value_