[面向对象] 一个关于继承的问题

module m_father
  implicit none
  type,abstract :: t_father
     integer :: a
   contains
     procedure(equation),pass(this),deferred :: equation
  end type t_father

  abstract interface
     module subroutine equation(this,other)
       implicit none
       class(t_father),intent(inout) :: this
       class(t_father),intent(in) :: other
     end subroutine
  end interface

  private equation
end module m_father

module m_son_1
  use m_father
  implicit none
  type,extends(t_father) :: t_son_1
     integer :: b
   contains
     procedure,pass(this) :: equation
     procedure,pass(this) :: equation_own

     generic,public :: assignment(=) => equation,equation_own
  end type t_son_1

  private equation
contains
  subroutine equation(this,other)
    implicit none
    class(t_son_1),intent(inout) :: this
    class(t_father),intent(in) :: other

    this%a=other%a
  end subroutine equation

  subroutine equation_own(this,other)
    implicit none
    class(t_son_1),intent(inout) :: this
    class(t_son_1),intent(in) :: other

    this%a=other%a
    this%b=other%b
  end subroutine equation_own
end module m_son_1

module m_son_2
  use m_father
  implicit none
  type,extends(t_father) :: t_son_2
     integer :: c
   contains
     procedure,pass(this) :: equation

     generic,public :: assignment(=) => equation
  end type t_son_2

  private equation
contains
  subroutine equation(this,other)
    implicit none
    class(t_son_2),intent(inout) :: this
    class(t_father),intent(in) :: other

    this%a=other%a
  end subroutine equation
end module m_son_2

program main
  use m_son_1
  use m_son_2
  implicit none

  type(t_son_1) :: a
  type(t_son_2) :: b
  b%a=1

  a=b
  call a%equation(b)
  print *,a%a

end program main