span class="identifier">T</span><span class="special">&gt;</span>
          <span class="keyword">constexpr</span> <span class="identifier">T</span>
          <span class="identifier">exchange</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">t</span><span class="special">,</span> <span class="identifier">U</span><span class="special">&amp;&amp;</span> <span class="identifier">u</span><span class="special">);</span></code></stron