/usr/include/boost/signals2/detail/ is in libboost1.58-dev 1.58.0+dfsg-5ubuntu3.
This file is owned by root:root, with mode 0o755.
..
/usr/include/boost/signals2/detail/auto_buffer.hpp
/usr/include/boost/signals2/detail/foreign_ptr.hpp
/usr/include/boost/signals2/detail/lwm_nop.hpp
/usr/include/boost/signals2/detail/lwm_pthreads.hpp
/usr/include/boost/signals2/detail/lwm_win32_cs.hpp
/usr/include/boost/signals2/detail/null_output_iterator.hpp
/usr/include/boost/signals2/detail/preprocessed_arg_type.hpp
/usr/include/boost/signals2/detail/preprocessed_arg_type_template.hpp
/usr/include/boost/signals2/detail/replace_slot_function.hpp
/usr/include/boost/signals2/detail/result_type_wrapper.hpp
/usr/include/boost/signals2/detail/signal_template.hpp
/usr/include/boost/signals2/detail/signals_common.hpp
/usr/include/boost/signals2/detail/signals_common_macros.hpp
/usr/include/boost/signals2/detail/slot_call_iterator.hpp
/usr/include/boost/signals2/detail/slot_groups.hpp
/usr/include/boost/signals2/detail/slot_template.hpp
/usr/include/boost/signals2/detail/tracked_objects_visitor.hpp
/usr/include/boost/signals2/detail/unique_lock.hpp
/usr/include/boost/signals2/detail/variadic_arg_type.hpp
/usr/include/boost/signals2/detail/variadic_slot_invoker.hpp