ABAP Widening Casting

REPORT  zthomas_oo_casting.
"class definition
*----------------------------------------------------------------------*
*       CLASS lcl_super DEFINITION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS lcl_super DEFINITION .
  PUBLIC SECTION.
    METHODS:add
            IMPORTING f_a TYPE i
                      f_b TYPE i
            EXPORTING f_c TYPE i.
ENDCLASS.                    "lcl_super DEFINITION
*----------------------------------------------------------------------*
*       CLASS lcl_super IMPLEMENTATION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS lcl_super IMPLEMENTATION.
  METHOD add.
    f_c = f_a + f_b.
  ENDMETHOD.                    "add
ENDCLASS.                    "lcl_super IMPLEMENTATION
*----------------------------------------------------------------------*
*       CLASS lcl_sub DEFINITION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS lcl_sub DEFINITION INHERITING FROM lcl_super.
  PUBLIC SECTION.
  METHODs add REDEFINITION.
  METHODs sub
              IMPORTING f_a TYPE i
                      f_b TYPE i
            EXPORTING f_c TYPE i..
ENDCLASS.                    "lcl_sub DEFINITION
*----------------------------------------------------------------------*
*       CLASS lcl_sub IMPLEMENTATION
*----------------------------------------------------------------------*
*
*----------------------------------------------------------------------*
CLASS lcl_sub IMPLEMENTATION.
METHOD add.
     f_c = f_a + f_b + 1.
ENDMETHOD.
METHOD sub.
     f_c = f_a - f_b .
ENDMETHOD.
ENDCLASS.                    "lcl_sub IMPLEMENTATION
START-OF-SELECTION.
  PARAMETERS:p_a TYPE i,
             p_b TYPE i.
  DATA:gv_add TYPE i,
       gv_sub TYPE i,
       ref1 TYPE REF TO lcl_sub,
       ref3 TYPE REF TO lcl_sub,
       ref2 TYPE REF TO lcl_super.
  CREATE OBJECT ref3.
  ref2 = ref3.
  ref1  ?= ref2.
  call METHOD ref1->add
                 EXPORTING f_a = p_a
                           f_b = p_b
                 IMPORTING f_c = gv_add.
  CALL METHOD ref1->sub
                   EXPORTING f_a = p_a
                           f_b = p_b
                 IMPORTING f_c = gv_sub.
WRITE:gv_add,gv_sub.


这个widening casting不能类比narrowing casting，具体分析见下图中sap bc404中解释