Yes, you declare the parameter as

    Flag!"async"

And then the possible values are Yes.async and No.async This exploits D dispatch operator: http://dlang.org/operatoroverloading.html#dispatch which in general is great for implementing this sort of dynamic feature in a strongly statically typed language as D - everything is checked as compile time!

You could look at the linked documentation. I think it makes it clear.

PS: I don't use D.