Fibonacci revisitated

In BOO, we can easily assign something like:

   a, b = b, a + b

a and b are assigned in two independent branches, without side effects. In languages like C#, we often have to use temporary variables to garantee isolation.

A friend of mine just, Tiago Epifânio, posted a comment referring a pretty cool way to do it without the temporary variable:

   b = a + (a = b);

Oops, wrong again. 2 years latter I had to rewrite it to:

   b = a + (a = b) - a;

<update>

This usage is not as fun as the swap snippet Tiago gave us on his first comment - given the right order and a subtle variable substitution, the branches don't side effect; apart from readability, note that we could always express it like:

   a = b;

   b = a + a;

</update>

Now that we're talking about isolation, it seems to me that BOO is actually doing something like:

   Stack<int> stack = new Stack<int>(2);
  
   stack.Push(b);
   stack.Push(a + b);
  
   b = stack.Pop();
   a = stack.Pop();

Oops, clumsy... The question is: is there a clean and simple way to express "a, b = b, a + b" in C#?