al">,</span> <span class="keyword">typename</span> Variant2<span class="special">&gt;</span> 
  <span class="keyword">typename</span> <span class="identifier">BinaryVisitor</span><span class="special">::</span><span class="identifier">result_type</span> <span class="identifier">OR</span> <span class="keyword">decltype</span><span class="special">(</span><span class="keyword">auto</span><span class="special">)</span> 
  <span class="identifier">apply_visitor</span><span class="special">(</span><span class="k