aspect NullObjects {
  // the actual unknown declaration null object is created in PredefinedTypes.jrag
    
  // explicit check if declaration is null object
  syn boolean Decl.isUnknown() = false;
  eq UnknownDecl.isUnknown() = true;


  // all types are compatible with the unknown type
  eq UnknownDecl.isSubtypeOf(TypeDecl typeDecl) = true;
  eq UnknownDecl.isSuperTypeOf(TypeDecl typeDecl) = true;
  eq UnknownDecl.isSuperTypeOfClassDecl(ClassDecl typeDecl) = true;

    
  // broadcast the unknown declaration null object to useful places in the AST
  syn lazy UnknownDecl Program.unknownDecl() = (UnknownDecl) localLookup("$unknown");
  eq Program.getBlock().unknownDecl() = unknownDecl();
  eq Program.getPredefinedType().unknownDecl() = unknownDecl();

  inh Decl TypeDecl.unknownDecl();
  inh Decl Block.unknownDecl();
}