/usr/include/boost/signals2/ is in libboost1.62-dev 1.62.0+dfsg-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/boost/signals2/connection.hpp
/usr/include/boost/signals2/deconstruct.hpp
/usr/include/boost/signals2/deconstruct_ptr.hpp
/usr/include/boost/signals2/detail/
/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
/usr/include/boost/signals2/dummy_mutex.hpp
/usr/include/boost/signals2/expired_slot.hpp
/usr/include/boost/signals2/last_value.hpp
/usr/include/boost/signals2/mutex.hpp
/usr/include/boost/signals2/optional_last_value.hpp
/usr/include/boost/signals2/postconstructible.hpp
/usr/include/boost/signals2/predestructible.hpp
/usr/include/boost/signals2/preprocessed_signal.hpp
/usr/include/boost/signals2/preprocessed_slot.hpp
/usr/include/boost/signals2/shared_connection_block.hpp
/usr/include/boost/signals2/signal.hpp
/usr/include/boost/signals2/signal_base.hpp
/usr/include/boost/signals2/signal_type.hpp
/usr/include/boost/signals2/slot.hpp
/usr/include/boost/signals2/slot_base.hpp
/usr/include/boost/signals2/trackable.hpp
/usr/include/boost/signals2/variadic_signal.hpp
/usr/include/boost/signals2/variadic_slot.hpp