title: Value initialization of POD struct is a constexpr?
body: <p>Consider the struct:</p>
  
  <pre><code>struct mystruct { };
  </code></pre>
  
  <p>Is it true that this is always valid:</p>
  
  <pre><code>constexpr mystruct mystructInstance = mystruct();
  </code></pre>
  
  <p>i.e. that value initialization of POD is a <code>constexpr</code>?  Similarly how about if the struct is defined to be:</p>
  
  <pre><code>struct mystruct { ~mystruct(); };
  </code></pre>
  
  <p>Finally, what about this:</p>
  
  <pre><code>struct mystruct { mystruct(); ~mystruct(); };
  </code></pre>
  
  <p>I haven't declared the ctr as constexpr, however are there any implicit deduction rules which guaratee this?</p>