title: How to define a constant double at namespace scope with external linkage?
body: <p>I am trying to create a <strong>namespace-scope</strong> constant with <strong>external linkage</strong></p>
  
  <pre><code>// in some include file:
  
  namespace foo 
  {
      constexpr double bar() { return 1.23456; } // internal linkage
      constexpr double baz = 1.23456;            // internal linkage
      const double bing = 1.23456;               // internal linkage
  }
  </code></pre>
  
  <p>Is this even possible?</p>