Covariant Returns and Tasks

[TonyValenti]

How do you use the two together?

    public class DerivedClass : BaseClass {
        //This works
        public override DerivedClass This() {
            return this;
        }

        //This does not.
        public override Task<DerivedClass> ThisAsync() {
            return Task.FromResult(this);
        }
    }

    public class BaseClass {
        public virtual BaseClass This() {
            return this;
        }

        public virtual Task<BaseClass> ThisAsync() {
            return Task.FromResult(this);
        }
    }

[huoyaoyuan]

They cannot be used together right now, because Task<T> isn't covariant.
Somehow related: #3950

[svick]

See also dotnet/roslyn#2981 on previous discussion about covariance of tasks, which would enable this.