Call overridden procedure of abstract parent type

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15

An alternative workaround is as follows:

   ..
   type, abstract, public :: parent_t
   contains
      private
      procedure, public :: say_hello_default
      procedure, public :: say_hello => say_hello_default
   end type
   ..

This can help make it easier to work with extended types in certain situations e.g., further subclassing of child_t in a different module (and which may be in another program package altogether) with possibly no access to module hosting the parent abstract “class”. Note this workaround is based on the flexibility in the Fortran language standard where 2 or more binding-names of type-bound procedures of a derived type can point to the same specific procedure-name. Extended types then override the binding-name corresponding to a “generic name” (say_hello, init, etc.), leaving the default name as-is.

Here`s how it can look with the MWE in the original post:

module parent_m
   implicit none
   private

   type, abstract, public :: parent_t
   contains
      private
      procedure, public :: say_hello_default
      procedure, public :: say_hello => say_hello_default
   end type
contains
   subroutine say_hello_default(self)
      class(parent_t), intent(in) :: self

      print *, "Hello from parent_t"
   end subroutine
end module

module child_m
   use parent_m, only: parent_t

   implicit none
   private

   type, extends(parent_t), public :: child_t
   contains
      private
      procedure, public :: say_hello => say_hello_child !<-- override the "generic"
   end type
contains
   subroutine say_hello_child(self)
      class(child_t), intent(in) :: self

      print *, "Hello from child_t"
      call self%say_hello_default()  !<-- invoke the base method as needed
   end subroutine
end module

program main
   use child_m, only: child_t

   implicit none

   type(child_t) :: child

   call child%say_hello()
end program

C:\Temp>ifort /standard-semantics /warn:all /stand:f18 p.f90
Intel(R) Fortran Intel(R) 64 Compiler Classic for applications running on Intel(R) 64, Version 2021.1.2 Build 20201208_000000
Copyright (C) 1985-2020 Intel Corporation. All rights reserved.

p.f90(12): remark #7712: This variable has not been used. [SELF]
subroutine say_hello_default(self)
--------------------------------^
Microsoft (R) Incremental Linker Version 14.26.28806.0
Copyright (C) Microsoft Corporation. All rights reserved.

-out:p.exe
-subsystem:console
p.obj

C:\Temp>p.exe
Hello from child_t
Hello from parent_t

C:\Temp>