title: Conditionally constexpr member function
body: <p>Suppose I have a template class</p>
  
  <pre><code>template &lt;typename T&gt;
  class foo {
      T m;
  
      decltype(auto) f() { return m.f(); }
  };
  </code></pre>
  
  <p>How can I give <code>foo:f()</code> the constexpr specifier only if <code>T::f()</code> is constexpr?</p>