title: Determine `constexpr` execution - during compilation or at runtime?
body: <p>Is there a way to achieve different behaviour of a <code>constexpr</code> function in the compilation phase and at runtime?</p>
  
  <p>Consider the following example (using a theoretical feature from <code>D: static if</code>):</p>
  
  <pre><code>constexpr int pow( int base , int exp ) noexcept
  {
      static if( std::evaluated_during_translation() ) {
          auto result = 1;
          for( int i = 0 ; i &lt; exp ; i++ )
              result *= base;
          return result;
      } else { // std::evaluated_during_runtime()
          return std::pow( base , exp );
      }
  }
  </code></pre>
  
  <p>If not, is there a way to restrict <code>constexpr</code> to be compile-time only?</p>