Re: Is this actually supposed to be legal?

[Tommi]

There's a thorough explanation of how "incomplete types" work in 
C++:
http://www.drdobbs.com/the-standard-librarian-containers-of-inc/184403814
And there's some more C++ related stuff:
http://www.boost.org/doc/libs/1_50_0/doc/html/container/containers_of_incomplete_types.html

I wouldn't know about D though.