title: Equivalent is_constructible to capture protected construction in derived
body: <p>Like said in the title. 
  Would anyone be able to propose an equivalent to std::is_constructible that would succeed in testing construction of an hypothetical derived class using a protected constructor of the tested class?</p>
  
  <pre><code>class A {
  protected:
       A(int);
  };
  static_assert(std::is_constructible&lt;A,int&gt;::value,"fails but ok");
  static_assert(is_derived_constructible&lt;A,int&gt;::value,"would work");
  </code></pre>