title: is_defined constexpr function
body: <p>I need to know whether NDEBUG is defined when specifying the noexcept specifier. I was thinking along the lines of this constexpr function:</p>
  
  <pre><code>constexpr inline bool is_defined() noexcept
  {
    return false;
  }
  
  constexpr inline bool is_defined(int) noexcept
  {
    return true;
  }
  </code></pre>
  
  <p>Then use it like:</p>
  
  <pre><code>void f() noexcept(is_defined(NDEBUG))
  {
    // blah, blah
  }
  </code></pre>
  
  <p>Does the standard library or the language already provide a facility for this so that I won't be reinventing the wheel?</p>