Undefined symbols for deferred bindings

["Rouson, Damian" <rouson at sandia dot gov>]

The code below compiles correctly (hooray!), but the linker can't find the
symbols related to the deferred bindings.  It's possible there's a problem
in my link line, so you might jump straight to the bottom.  Otherwise, foo
defines an abstract type (as in my previous posting); bar extends foo and
overrides foo's deferred bindings; and main declares a bar object and passes
it to the "rescale" procedure inside foo_module.

Damian

$ cat foo.f90 
module foo_module
  implicit none
  private
  public :: foo,rescale

  type ,abstract :: foo
  contains
    procedure(times_interface) ,deferred :: times
    procedure(assign_interface) ,deferred :: assign
    generic :: operator(*) => times
    generic :: assignment(=) => assign
  end type

  abstract interface
    function times_interface(this,factor) result(product)
      import :: foo
      class(foo) ,intent(in) :: this
      class(foo) ,allocatable :: product
      real, intent(in) :: factor
    end function 
    subroutine assign_interface(lhs,rhs)
      import :: foo
      class(foo) ,intent(inout) :: lhs
      class(foo) ,intent(in) :: rhs
    end subroutine
  end interface

contains
  subroutine rescale(this,scale)
    class(foo) ,intent(inout) :: this
    real, intent(in) :: scale
    this = this*scale
  end subroutine
end module
$
$ cat bar.f90
module bar_module
  use foo_module ,only : foo
  implicit none
  private
  public :: bar

  type ,extends(foo) :: bar
    private
    real :: x=1.
  contains
    procedure :: times => times_bar
    procedure :: assign => assign_bar
  end type

contains
  subroutine assign_bar(lhs,rhs)
    class(bar) ,intent(inout) :: lhs
    class(foo) ,intent(in) :: rhs
    select type(rhs)
      type is (bar)
        lhs%x = rhs%x
    end select
  end subroutine
  function times_bar(this,factor) result(product)
    class(bar) ,intent(in) :: this
    real, intent(in) :: factor
    class(foo), allocatable :: product
    select type(this)
      type is (bar)
        allocate(product,source=this)
        select type(product)
          type is(bar)
            product%x = this%x*factor
        end select
    end select
  end function
end module
$
$ cat main.f90
program main
  use foo_module ,only : foo,rescale
  use bar_module ,only : bar
  implicit none
  type(bar) :: unit
  call rescale(unit,3.141592654)
end program
$
$ make
gfortran -c foo.f90
gfortran -c bar.f90
gfortran -c main.f90
gfortran main.o foo.o bar.o -o foobar
Undefined symbols:
  "_assign_interface_", referenced from:
      ___foo_module_MOD_rescale in foo.o
  "_times_interface_", referenced from:
      ___foo_module_MOD_rescale in foo.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
make: *** [foobar] Error 1