span class="identifier">compressed_pair</span></code>
      can be instantiated for reference and array types, however in these cases the
      range of constructors that can be used are limited. If types <code class="computeroutput"><span class="identifier">T1</span></code> and <code class="computeroutput"><span class="identifier">T2</span></code>
      are the same type, then there is only one version of the single-argument constructor,
      and this constructor initialises both values in the pair to the