Passing a UDT/struct between Fortran and C++

This is a follow on to my previous question (https://software.intel.com/en-us/forums/topic/534413), I am able to pass a simple data type as a return value, but what I really want to do is pass a user-defined type back to the Fortran as the return value from the C++ code.  I have a copy of "Modern Fortran Explained" and it suggests that this is indeed possible, but I am getting a compilation error saying that my derived type definition is unknown at the point where I define my interface to the C++-routine. 

Here is a sample Fortran code illustrating what I am trying to do:

program FortranCPPInterop

    type, bind(c) :: inter_op
        integer :: i
        double precision :: j
    end type inter_op

    interface
        function CPP_func(ARG)
        !DEC$ ATTRIBUTES C, DLLIMPORT, ALIAS:'_CPP_func' :: CPP_func
        !DEC$ ATTRIBUTES REFERENCE :: ARG
        TYPE(inter_op) :: CPP_func
        integer ARG
        end function CPP_func
    end interface

    TYPE(inter_op) :: test

    test = CPP_func( 3)

    print*, test%i
    print*, test%j
end program

And for completeness (again) the C++ routine it is calling:

struct interop_type {
	int i;
	double j;
};
extern "C" {
	__declspec(dllexport) interop_type CPP_func(int* arg) {
		interop_type ret_val;

		ret_val.i = *arg * 4;
		ret_val.j = *arg / 2.0;

		return ret_val;
	}
}

This set up works just fine if I tweak the C++ to return a basic type (an int), but when I try to have it return the struct/UDT, I get the following compilation errors:

Source1.f90(12): error #6457: This derived type name has not been declared.   [INTER_OP]
Source1.f90(19): error #6303: The assignment operation or the binary expression operation is invalid for the data types of the two operands.   [CPP_FUNC]

It seems pretty clear that the second error follows from the first, but I have no idea why I am getting the first error, as my type is defined.  Can anyone let me know what I might be missing?  Thank you!