title: constexpr int and constexpr double in c++
body: <p>I have encountered a strange case. </p>
  
  <pre><code>// this does not work, and reports:
  // constexpr variable 'b' must be initialized by a constant expression
  int main() {
    const double a = 1.0;
    constexpr double b = a;
    std::cout &lt;&lt; b &lt;&lt; std::endl;
  }
  
  // this works...
  int main() {
    const int a = 1;
    constexpr int b = a;
    std::cout &lt;&lt; b &lt;&lt; std::endl;
  }
  </code></pre>
  
  <p>anything special about <code>double</code> so it cannot make <code>constexpr</code> work?</p>