std::map<string, int> crashes when doing m[0] : <p>The following crashes:</p>

<pre><code>  std::map&lt;std::string, int&gt; m1{ {"0", 0}, { "1", 1 }};
  // auto melem = m1["0"]; // OK
  auto melem = m1[0];
</code></pre>

<p>Why is that?</p>
