how to make interface with return auto

[Eko Wahyudin via Digitalmars-d-learn]

I know it's imposible.

is there's a way if i want to create an interface that have 
method with return type value is equals with return type of 
std.palallelism.task

eg.
interface IWorker{
    auto activate(); //on implementation class activate return 
std.parallelism.task
}

What is the replacement of auto on that interface?

Thank you!