Why are constructor definitions preserved in interface files?

[Max Samukha via Digitalmars-d-learn]

module lib.a;

class C {
        this() {
        }

        void foo() {
        }

        ~this() {
        }

}


dmd -H -o- a.d:

// D import file generated from 'a.d'
module lib.a;
class C
{
        this()
        {
        }
        void foo();
        ~this();
}

The destructor and member function definitions have been removed 
as expected. What is the reason for keeping the constructor 
definition?