 Each overload
    generated constructs an object of <code>HeldType</code> according to the
    semantics described <a href="#HeldType">above</a>, using a copy of
    <code>init_expr</code>'s <a href="CallPolicies.html">call policies</a>.
    If the longest <a href="init.html#init-expressions">valid prefix</a> of
    <code>Init</code> contains <em>N</em> types and <code>init_expr</code>
    holds <em>M</em> keywords, an initial sequence of the keywords are used
    for all but the first <em>N</em> - <em>M</