Oracle 11g Release 2

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--A
DECLARE
  TYPE studentcur_t IS REF CURSOR RETURN DUAL%ROWTYPE;
  TYPE teachercur_t IS REF CURSOR;
  
  cursor1 studentcur_t;
  cursor2 teachercur_t;
  cursor3 SYS_REFCURSOR;
  
  CURSOR stcur IS SELECT * FROM DUAL;
BEGIN 
  OPEN cursor3 FOR SELECT * FROM DUAL;
  cursor1 :=cursor3;
END;
/
1 rows affected

--B
DECLARE
  TYPE studentcur_t IS REF CURSOR RETURN DUAL%ROWTYPE;
  TYPE teachercur_t IS REF CURSOR;
  
  cursor1 studentcur_t;
  cursor2 teachercur_t;
  cursor3 SYS_REFCURSOR;
  
  CURSOR stcur IS SELECT * FROM DUAL;
BEGIN 
  OPEN stcur;
  cursor1 :=stcur;
END;
/
ORA-06550: line 13, column 13:
PLS-00382: expression is of wrong type
ORA-06550: line 13, column 3:
PL/SQL: Statement ignored

--C
DECLARE
  TYPE studentcur_t IS REF CURSOR RETURN DUAL%ROWTYPE;
  TYPE teachercur_t IS REF CURSOR;
  
  cursor1 studentcur_t;
  cursor2 teachercur_t;
  cursor3 SYS_REFCURSOR;
  
  CURSOR stcur IS SELECT * FROM DUAL;
BEGIN 
  OPEN cursor1 FOR SELECT * FROM DUAL;
  stcur :=cursor1;
END;
/
ORA-06550: line 13, column 3:
PLS-00321: expression 'STCUR' is inappropriate as the left hand side of an assignment statement
ORA-06550: line 13, column 3:
PL/SQL: Statement ignored

--D
DECLARE
  TYPE studentcur_t IS REF CURSOR RETURN DUAL%ROWTYPE;
  TYPE teachercur_t IS REF CURSOR;
  
  cursor1 studentcur_t;
  cursor2 teachercur_t;
  cursor3 SYS_REFCURSOR;
  
  CURSOR stcur IS SELECT * FROM DUAL;
BEGIN 
  OPEN stcur;
  cursor3 :=stcur;
END;
/
ORA-06550: line 13, column 13:
PLS-00382: expression is of wrong type
ORA-06550: line 13, column 3:
PL/SQL: Statement ignored

--E
DECLARE
  TYPE studentcur_t IS REF CURSOR RETURN DUAL%ROWTYPE;
  TYPE teachercur_t IS REF CURSOR;
  
  cursor1 studentcur_t;
  cursor2 teachercur_t;
  cursor3 SYS_REFCURSOR;
  
  CURSOR stcur IS SELECT * FROM DUAL;
BEGIN 
  OPEN cursor1 FOR SELECT * FROM DUAL;
  cursor2 :=cursor1;
END;
/
1 rows affected