Casting one struct pointer to another - C

Please consider the following code.

enum type {CONS, ATOM, FUNC, LAMBDA};

typedef struct{
  enum type type;
} object;

typedef struct {
  enum type type;
  object *car;
  object *cdr;
} cons_object;

object *cons (object *first, object *second) {
  cons_object *ptr = (cons_object *) malloc (sizeof (cons_object));
  ptr->type = CONS;
  ptr->car = first;
  ptr->cdr = second;
  return (object *) ptr;
}

In the cons function, variable ptr is of type cons_object*. But in the return value it is converted to type of object*.

1. I am wondering how this is possible because cons_object and object are different structs.
2. Are there any issues in doing stuff like this?

Any thoughts!