CT-String as a Symbol

[via Digitalmars-d-learn]

On Wednesday, 15 April 2015 at 23:02:32 UTC, Ali Çehreli wrote:
>  struct I {
>                   alias T = size_t;
>                   this(T ix) { this._ix = ix; }
>                   T opCast(U : T)() const { return _ix; }
>                   private T _ix = 0;
>               }

How is this possible? Shouldn't it CT-evaluate to

     struct Index { ... }

!?