ALTER SESSION SET plsql_warnings = 'enable:all'

CREATE OR REPLACE FUNCTION my_function  
   RETURN BOOLEAN AUTHID DEFINER 
IS 
BEGIN 
   RETURN FALSE; 
END;

CREATE OR REPLACE FUNCTION my_date 
   RETURN DATE AUTHID DEFINER 
IS 
BEGIN 
   RETURN SYSDATE - 10; 
END;

CREATE OR REPLACE FUNCTION no_return (check_in IN BOOLEAN)
   RETURN VARCHAR2
AS
BEGIN
   IF check_in
   THEN
      RETURN 'abc';
   ELSE
      DBMS_OUTPUT.put_line ('Here I am, here I stay');

      IF my_function ()
      THEN
         RETURN 'def';
      ELSIF SYSDATE < my_date ()
      THEN
         RETURN 'qrs';
      ELSE
         DBMS_OUTPUT.put_line ('Hello!');
      END IF;
   END IF;
END no_return;

select text from user_errors 
 where name = 'NO_RETURN' 

BEGIN
   DBMS_OUTPUT.put_line (no_return (FALSE));
END;

ALTER SESSION SET plsql_warnings = 'ERROR:5005'

ALTER FUNCTION no_return COMPILE

CREATE OR REPLACE FUNCTION no_return (check_in IN BOOLEAN)
   RETURN VARCHAR2
AS
BEGIN
   IF check_in
   THEN
      RETURN 'abc';
   ELSE
      DBMS_OUTPUT.put_line ('Here I am, here I stay');

      IF check_in
      THEN
         RETURN 'def';
      ELSIF SYSDATE IS NOT NULL
      THEN
         RETURN 'qrs';
      ELSE
         DBMS_OUTPUT.put_line ('Hello!');
         RETURN 'hello';
      END IF;
   END IF;
EXCEPTION
   WHEN OTHERS
   THEN
      NULL;
END no_return;

CREATE OR REPLACE FUNCTION no_return (check_in IN BOOLEAN)
   RETURN VARCHAR2
AS
BEGIN
   RETURN NULL;
EXCEPTION
   WHEN OTHERS
   THEN
      RAISE NO_DATA_FOUND;
END no_return;

CREATE OR REPLACE FUNCTION error5005
   RETURN INTEGER
IS
BEGIN
   RETURN 5;
EXCEPTION
   WHEN OTHERS
   THEN
      raise_application_error (-20001, 'Error 5005');
END;

CREATE OR REPLACE FUNCTION error5005 
   RETURN INTEGER 
IS 
BEGIN 
   RETURN 5; 
EXCEPTION 
   WHEN OTHERS 
   THEN 
      raise_application_error (-20001, 'Error 5005'); 
END;

CREATE OR REPLACE FUNCTION no_return (check_in IN BOOLEAN) 
   RETURN VARCHAR2 
AS 
   l_return   VARCHAR2 (32767); 
BEGIN 
   IF check_in 
   THEN 
      l_return := 'abc'; 
   ELSE 
      DBMS_OUTPUT.put_line ('Here I am, here I stay'); 
 
      IF check_in 
      THEN 
         l_return := 'def'; 
      ELSIF SYSDATE IS NOT NULL 
      THEN 
         l_return := 'qrs'; 
      ELSE 
         DBMS_OUTPUT.put_line ('Hello!'); 
         l_return := 'hello'; 
      END IF; 
   END IF; 
 
   RETURN l_return; 
EXCEPTION 
   WHEN OTHERS 
   THEN 
      -- In theory: log_error (); 
      RAISE; 
END no_return;