interface SinkString
{
    void toString(void delegate(string) sink);
}

class Aggregate : SinkString
{
    override void toString(void delegate(string) @safe sink) @safe
    {
        sink("I'm an Aggreate!");
    }
}

Here, void delegate(string) @safe is not a super-type of void delegate(string) which it would need to be to be sound by the Liskov substitution principle. To the contrary, it allows passing @system `sink`s to an Aggregate object through a SinkString reference.