title: Hana BOOST_HANA_DEFINE_STRUCT does not work with std::unique_ptr
body: <p>Boost Hana's BOOST_HANA_DEFINE_STRUCT does not seem to work with std::unique_ptr as fields. Any workaround?</p>
  
  <pre><code>#include &lt;boost/hana.hpp&gt;
  #include &lt;memory&gt;
  
  struct Test
  {
      BOOST_HANA_DEFINE_STRUCT(Test,
          (unsigned, field0),
          (std::unique_ptr&lt;unsigned&gt;, field1));
  };
  
  int main(int argc, char** argv)
  {
      Test test;
      boost::hana::for_each(boost::hana::members(test), [&amp;](auto field)
      {
      });
  
      return 0;
  }
  </code></pre>
  
  <p>error: no matching constructor for initialization of 'tuple::type, typename detail::decay > &amp;>::type>' (aka 'tuple > >')
          { return {static_cast(xs)...}; }</p>