create or replace package PKG_TEST is

  procedure PrintTwo;

--THE PACKAGE FAILS TO COMPILE (WITH A PLS-00231 ERROR)
--WHEN THEN FOLLOWING LINE IS COMMENTED OUT,
--BUT COMPILES SUCCESSFULLY WHEN IT IS UNCOMMENTED.
  function Two return Number;

end PKG_TEST;
/
create or replace package body PKG_TEST is

  function Two
  return Number
  is
  begin
    return 2;
  end Two;
  
  procedure PrintTwo
  is
    myNum Number;
  begin
    select Two
    into myNum
    from dual;

    dbms_output.put_line(myNum);
  end PrintTwo;

end PKG_TEST;
/

  1  create or replace package PKG_TEST is
  2    procedure PrintTwo;
  3  --THE PACKAGE FAILS TO COMPILE (WITH A PLS-00231 ERROR)
  4  --WHEN THEN FOLLOWING LINE IS COMMENTED OUT,
  5  --BUT COMPILES SUCCESSFULLY WHEN IT IS UNCOMMENTED.
  6  --  function Two return Number;
  7* end PKG_TEST;
  8  /

Package created.


  1  create or replace package body PKG_TEST is
  2    function Two
  3    return Number
  4    is
  5    begin
  6      return 2;
  7    end Two;
  8    procedure PrintTwo
  9    is
 10      myNum Number;
 11    begin
 12      select Two()
 13      into myNum
 14      from dual;
 15      dbms_output.put_line(myNum);
 16    end PrintTwo;
 17* end PKG_TEST;
 18  /

Warning: Package Body created with compilation errors.

SQL> show errors
Errors for PACKAGE BODY PKG_TEST:

LINE/COL ERROR
-------- -----------------------------------------------------------------
12/5     PL/SQL: SQL Statement ignored
12/12    PL/SQL: ORA-00904: : invalid identifier
12/12    PLS-00231: function 'TWO' may not be used in SQL
SQL> ed
Wrote file afiedt.buf

  1  create or replace package body PKG_TEST is
  2    function Two
  3    return Number
  4    is
  5    begin
  6      return 2;
  7    end Two;
  8    procedure PrintTwo
  9    is
 10      myNum Number;
 11    begin
 12  --    select Two()
 13  --    into myNum
 14  --    from dual;
 15        myNum := Two();
 16      dbms_output.put_line(myNum);
 17    end PrintTwo;
 18* end PKG_TEST;
SQL> /

Package body created.

SQL>