title: Submodules: how to divide C++ module into submodules
body: <p>I have a simple project with one module.</p>
  <p>Units.ixx</p>
  <pre><code>module;
  
  #include &lt;cassert&gt;
  export module Units;
  export class Base {
  public:
  };
  export void foo();
  </code></pre>
  <p>Units_impl.ixx</p>
  <pre><code>export module Units;
  
  class Derived : public Base {  // 'Base': base class undefined
  };
  
  export void foo() {
      Derived derived;
  }
  </code></pre>
  <p>Main.cpp</p>
  <pre><code>import Units;
  
  int main() {
      return foo();
  }
  </code></pre>
  <p>Compiler shows me an error 'Base': base class undefined.
  Is it expected behavior? If yes, how to divide modules into submodules? I’d like to have Derived inside my Units module but in a different file.</p>