no size yet for forward reference error

[Marc Schütz via Digitalmars-d]

Digger shows that it stopped working after this PR:
https://github.com/D-Programming-Language/dmd/pull/4457

I could reduce it as far as this:

struct RefCounted(T) {
     struct Impl {
         T _payload;
     }

     Impl* _store;

     ~this() {
         destroy(_store._payload);
     }
}

struct Connection(T) {
     alias Statement = .Statement!T;
}

struct Statement(T) {
     alias Connection = .Connection!T;

     struct Payload {
         Connection con;
     }

     RefCounted!Payload Data;
}

Connection!int x;