Thanks to Sogo Mineo (NAOJ) who identified the bug through gcc 8.1 and
provided the fix.

The code in the removed block that not already been #ifdef'd
out was rife with serious errors that had gone unnoticed until
recently because they were in templates that were not being
instantiated (because they were only used in code that had
been #ifdef'd out).

IdSpan is barely larger than shared_ptr itself, and
holding it by value avoids both an extra level of
indirection and a lot of thread-atomic reference counting.