title: A constexpr function is not required to return a constant expression?
body: <p><strong>C++ Primer</strong> (5th edition) on page 240 has a note stating: </p>
  
  <blockquote>
    <p>"A <code>constexpr</code> function is not required to return a constant expression". </p>
  </blockquote>
  
  <p>A question has been asked about this: <a href="https://stackoverflow.com/questions/29577778/can-constexpr-function-return-type-be-a-non-const">can constexpr function return type be a non const?</a>. The author of that question misunderstood the note. </p>
  
  <p>But what is the correct understanding of it (the answers to the cited post clarify the confusion of that post's author, but do not answer my question)?</p>