/usr/include/boost/fiber/detail/ is in libboost1.65-dev 1.65.1+dfsg-0ubuntu5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/boost/fiber/detail/config.hpp
/usr/include/boost/fiber/detail/context_spinlock_queue.hpp
/usr/include/boost/fiber/detail/context_spmc_queue.hpp
/usr/include/boost/fiber/detail/convert.hpp
/usr/include/boost/fiber/detail/cpu_relax.hpp
/usr/include/boost/fiber/detail/data.hpp
/usr/include/boost/fiber/detail/decay_copy.hpp
/usr/include/boost/fiber/detail/disable_overload.hpp
/usr/include/boost/fiber/detail/fss.hpp
/usr/include/boost/fiber/detail/futex.hpp
/usr/include/boost/fiber/detail/rtm.hpp
/usr/include/boost/fiber/detail/spinlock.hpp
/usr/include/boost/fiber/detail/spinlock_rtm.hpp
/usr/include/boost/fiber/detail/spinlock_status.hpp
/usr/include/boost/fiber/detail/spinlock_ttas.hpp
/usr/include/boost/fiber/detail/spinlock_ttas_adaptive.hpp
/usr/include/boost/fiber/detail/spinlock_ttas_adaptive_futex.hpp
/usr/include/boost/fiber/detail/spinlock_ttas_futex.hpp