Lazy values in D

struct Lazy(T) {
  bool forced = false;

  // Ideally thunk and value would be part of a union
  // but I don't know if that would work with D's GC.
  T function() thunk;

  T value;

  this(T t) { thunk = T function() { return t; }; }

  @property T force() {
    if (!forced) {
      value = thunk();

      // thunk should now be garbage
      thunk = null;
    }

    return value;
  }

  alias force this;
}