[tdpl] There can be only one alias this.

From TDPL, page 231: "A class could introduce any number of alias this declarations, thus subtyping any number of types." Let's test this: class A{} class B{} class C{ A a; B b; alias a this; alias b this; } Error: alias this there can be only one alias this (I think that error message has a very nice humorous touch!) The same applies to structs.