This is an implicit, GC-backed memory session. Implicit sessions cannot be closed explicitly. While it would be possible to model an implicit session as a non-closeable view of a shared session, it is better to capture the fact that an implicit session is not just a non-closeable view of some session which might be closeable. This is useful e.g. in the implementations of DirectBuffer#address(), where obtaining an address of a buffer instance associated with a potentially closeable session is forbidden.