title: Impossible multi-dimensional array in C
body: <p>I was reading some introductory material on programming in C.
  The following example was presented:</p>
  <pre><code>int matrix[][] = {{ 1,2,3}, {5,6,7}};
  </code></pre>
  <p>As gcc (7.5.0 in my case) and clang (6.0.0 in my case) did not compile, I tried several C standards (c89, c90), but none worked.</p>
  <p>Is there a C standard version in which this is valid?</p>