cast(A)b is not an lvalue

[Namespace]

If I don't comment out line 19 I get:
/home/c803/c821.d(19): Error: function c821.foo (ref A a) is not 
callable using argument types (B)
/home/c803/c821.d(19): Error: cast(A)b is not an lvalue

Code: http://dpaste.dzfl.pl/89f55c62

Should not work all three?