Until recently, libfolks-telepathy had this:

public class Tpf.PersonaStore : Folks.PersonaStore
{
  ...
  internal SmallSet<string> _supported_fields;
  internal Set<string> _supported_fields_ro;
  ...
}

When compiled without using --internal-vapi or --internal-header, this results in the C struct in the public header looking like this:

    struct _TpfPersonaStore {
            FolksPersonaStore parent_instance;
            TpfPersonaStorePrivate * priv;
            FolksSmallSet* _supported_fields;
            GeeSet* _supported_fields_ro;
    };

and the header from the .vapi for FolksSmallSet being #include'd.

Unfortunately, FolksSmallSet is declared in a header that isn't installed (it's a private C implementation of Gee.Set optimized for small or empty sets), so this broke the build of dependent projects.

Ideally, adding "internal Foo" to a class shouldn't result in changes to the public header (although I can't really see a way to to achieve that).