Can opApply be made @nogc?

[solidstate1991]

Since it's a bit difficult to make tree traversal through range 
(especially if someone wants to make it @nogc), I thought I'll 
make it through opApply override, however the delegate passed by 
it doesn't have the @nogc attribute, which would automatically 
make it incapable to be used in a @nogc context.

I also don't know if it would work with structs instead of 
classes, since they're easier to handle in a @nogc situation.