title: std::pair of references
body: <p>Is it valid to have a <code>std::pair</code> of references ? In particular, are there issues with the assignment operator ? According to <a href="http://www.cplusplus.com/reference/std/utility/pair/" rel="noreferrer">this link</a>, there seems to be no special treatment with operator=, so default assignement operator will not be able to be generated.</p>
  
  <p>I'd like to have a <code>pair&lt;T&amp;, U&amp;&gt;</code> and be able to assign to it another pair (of values or references) and have the pointed-to objects modified.</p>