title: Unordered_map constructed from initializer list with repeated keys
body: <p>What does the standard guarantee will happen if an <code>unordered_map</code> (or more generally a unique associative container) is constructed with an <code>initializer_list</code> with repeated keys? Is it undefined or does it have unique compliant behavior? gcc 4.7 accepts code like this</p>
  
  <pre><code> std::unordered_map&lt;int, int&gt; x = {{1,1}, {1,2}};
  </code></pre>
  
  <p>but that doesn't tell me what is guaranteed by the standard.</p>