std.variant.Algebraic, self referential types and delegate members

[Panke via Digitalmars-d-learn]

import std.variant, std.stdio;

---
struct NodeTypeA(T) { T[] children; }
struct NodeTypeB(T) { Tree children; }
struct Leaf(T) { T delegate() dg; }

alias Tree = Algebraic!(Leaf, NodeTypeA!This, NodeTypeB!This);

void main()
{
   Tree t;
}
---

yields

tmp.d(6): Error: functions cannot return opaque type This by value
tmp.d(8): Error: template instance tmp.Leaf!(This) error 
instantiating

This limitation seems arbitrary to me. What's the reason here?