The uniform structure filter_arg sets its union based on the difference
of enum filter_arg_type, However, some functions use implicit type
conversion obviously.
warning: implicit conversion from enumeration type 'enum filter_exp_type'
to different enumeration type 'enum filter_op_type'
warning: implicit conversion from enumeration type 'enum filter_cmp_type'
to different enumeration type 'enum filter_exp_type'