title: Why is the std::bitset constructor with an unsigned long long argument not marked as explicit?
body: <p>The Standard Library class template <code>std::bitset&lt;N&gt;</code> has a constructor (C++11 and onwards, <code>unsigned long</code> argument before C++11)</p>
  
  <pre><code>constexpr bitset(unsigned long long) noexcept 
  </code></pre>
  
  <p>Contrary to many best-practice guidelines, this single-argument constructor is not marked as <code>explicit</code>. What is the rationale behind this?</p>