title: Can virtual functions be constexpr?
body: <p>Can virtual functions like <code>X::f()</code> in the following code</p>
  
  <pre><code>struct X 
  {
      constexpr virtual int f() const 
      {
          return 0;
      }
  };
  </code></pre>
  
  <p>be <code>constexpr</code>?</p>