why are Assign Expressions r-values?

[Manfred Nowak]

BCS wrote

> however the expression could be used as an l-value

Because one can overload the assignment, your question has a 
generalization:

given a function with exactly one "ref" or "out" parameter, why is this 
function an r-value?

example:
  int f( int i, ref int j){ }

now the semantics of an assignment like

  f( a, f( b, c))= d;

should be clear.

-manfred