title: how to only enable this function for string or stringpiece
body: <p>I have a template function, let's say:</p>
  
  <pre><code>template &lt;typename T&gt;
  void foo(T input) {
      // some funny processing
  }
  </code></pre>
  
  <p>I only want to enable this function for T == string or T == stringpiece.
  How do I do that using std::enable_if ???</p>