title: deducing return type to be type of template arguments method
body: <p>It's hard for me to explain in English exactly what I mean, but the following non-compilable code might illustrate what I'm after:</p>
  
  <pre><code>template&lt;class T&gt;
  auto fn(T t) -&gt; decltype(T::method_call())
  {
      return t.method_call();
  }
  </code></pre>
  
  <p>Basically I want the function to return whatever it is T's method returns. What's the syntax to achieve this?</p>