$ cat vector.d #-----------------------------------------------------
module vector;

class Vector(E) {
  E[] data;
}

$ cat student.d #-----------------------------------------------------
module student;

import vector;
import teacher;

// define Student
class Student {
  void ask(Teacher teacher) {
  }
}

// define Students
alias Vector!(Student) Students;

$ cat teacher.d #-----------------------------------------------------
module teacher;

import student;

class Teacher {
  Students students;
}

===============================================
$ dmd -c student.d 
teacher.d(6): Error: forward reference to 'Vector!(Student)'
teacher.d(6): Error: Students is used as a type
teacher.d(6): variable teacher.Teacher.students voids have no value

===============================================
sure I know if I do:
$ dmd -c vector.d  teacher.d student.d
all the three files can be compiled without error.

But my question is: why can't file be individually compiled?

I think I have the most natural/logical organization of files; if you move the 'Students' alias around, it could get compiled, but what's wrong with my current organization?