Why C++ copy constructor must use const object?

Question

I understand that when we define a class copy constructor of the class is necessary as Rule of three states. I also notice that the argument of the copy constructor is usual

Answer 1

In addition to the fundamental assumption that copy constructors should not modify the source instance, this article elaborates on the actual technical reason for using const:

http://www.geeksforgeeks.org/copy-constructor-argument-const/

Namely that and I quote:

"... compiler created temporary objects cannot be bound to non-const references ..."