title: inline namespace and extension namespace
body: <p>I read the section about namespace definition. Clause 7.3.1 of N3797 said:</p>
  
  <blockquote>
    <p>The inline keyword may be used on an extension-namespace-definition
    only if it was previously used on the original-namespace-definition
    for that namespace.</p>
  </blockquote>
  
  <p>Consider the following code snippet:</p>
  
  <pre><code>namespace M
  {
      int h;
  }
  
  inline namespace M
  {
      int j = 6;
  }
  </code></pre>
  
  <p>It compiled successful both with the <code>-std=c++11</code> and without that option. Can you explain that behavior? Is it a <code>g++</code> bug?</p>