title: Checking if variadic template parameters are unique using fold expressions
body: <p>Given a variadic template parameter pack, I want to check if all types given to it are unique using an <code>inline constexpr bool</code> and <a href="http://en.cppreference.com/w/cpp/language/fold" rel="nofollow noreferrer" title="fold expressions">fold expressions</a>. I trie something like this:</p>
  <pre><code>template&lt;class... T&gt;
  inline static constexpr bool is_unique = (... &amp;&amp; (!is_one_of&lt;T, ...&gt;));
  </code></pre>
  <p>Where <code>is_one_of</code> is a similar bool that works correctly.
  But this line doesn't compile regardless of what I put into is_one_of. Can this even be done using fold expressions, or do I need to use a regular struct for this purpose?</p>