n class="k">constexpr</span> <span class="n">T</span> <span class="k">operator</span><span class="p">()(</span><span class="n">X</span> <span class="o">&amp;&amp;</span><span class="n">v</span><span class="p">);</span>
<span class="p">};</span>
</code></pre></div>
<p><em>Overridable</em>: By template specialisation into the <code>convert</code> namespace.</p>

<p><em>Default</em>: If decayed <code>X</code> is same as <code>U</code>, concept <code>ValueOrError&lt;U&gt;</code> matches, <code>X::value_type</co