 is returned.</p>

<p><em>Requires</em>: <code>operator!=</code> must be a valid expression between <code>value_type</code> and <code>A</code>, and between <code>error_type</code> and <code>B</code>. If <code>value_type</code> is <code>void</code>, then so must be <code>A</code>; similarly for <code>error_type</code> and <code>B</code>.</p>

<p><em>Complexity</em>: Whatever the underlying <code>operator!=</code> have. Constexpr and noexcept of underlying operations is propagated.</p>

<p><em>Guarantees</em>