const ref in opAssign

[monarch_dodra]

I just finished reading the chapters on user defined types and, 
(as a C++ dev), this one line struc out to me as very odd:

"ref Widget opAssign(ref Widget rhs) {..}"

This means that during an assignment, I could potentially change 
"Other" (!). This seems like a blatant violation of the expected 
behavior of =. What's more, it prevents the assignment from a 
const object...

I am very tempted to change the call to "const ref". Is this 
un-advised?

Mr. Alexandrescu goes on to mention a "pass by value" in case you 
wanted to assign from a temporary, mentioning:
w = Widget(50); // Error!
// Cannot bind an rvalue of type Widget to ref Widget!

The only problem is that if I do this, all of my calls are then 
re-routed to pass by value, and none to the pass-by-const-ref.

----

Is there any way to enforce const correctness, while still 
keeping advantage of both calls?

PS: using Mr. Alexandrescu's design, it is not possible to assign 
from a const Widget, I get:
hello.d(50): Error: function hello.Widget.opAssign (Widget rhs) 
is not callable using argument types (const(Widget))
hello.d(50): Error: cannot implicitly convert expression (w2) of 
type const(Widget) to Widget

const can't be passed by value...?