title: What does the void() in decltype(void()) mean exactly?
body: <p>This is a follow-up of <a href="https://stackoverflow.com/questions/39278200/is-sizeofvoid-a-legal-expression">this</a> question, more precisely of the comments of <a href="https://stackoverflow.com/a/39278306/4987285">this</a> answer.</p>
  
  <p>What does the <code>void()</code> in <code>decltype(void())</code> represent exactly?<br>
  Does it represent a function type, an expression or whatever?</p>