I have the following code in a Foo.mojom file:

interface Foo {
  DoFoo(Baz.Bar bar);
}

interface Baz {
  enum Bar { ... };
}

That fails to compile because the generated function in foo.mojom.h has an incomplete type:

error: incomplete type 'Baz' named in nested name specifier
  virtual void DoFoo(Baz::Bar bar) = 0;

note: forward declaration of 'Baz'
class Baz;

Switching the order of the mojom file to declare Baz before Foo fixes the issue.