Is the compiler supposed to accept this?

[Brian Schott]

While finishing up work on my parser and grammar specification I 
found this in container.d:

return equal!(function(Elem a, Elem b) => !_less(a,b) && 
!_less(b,a))
                     (thisRange, thatRange);
It seems to be some strange hybrid of the function literal syntax 
and the lambda syntax. It's not documented anywhere (surprise!) 
and I'm not sure if I should support it or file an 
accepts-invalid bug against DMD.