/usr/include/boost/variant/detail/ is in libboost1.62-dev 1.62.0+dfsg-5.
This file is owned by root:root, with mode 0o755.
..
/usr/include/boost/variant/detail/apply_visitor_binary.hpp
/usr/include/boost/variant/detail/apply_visitor_delayed.hpp
/usr/include/boost/variant/detail/apply_visitor_unary.hpp
/usr/include/boost/variant/detail/backup_holder.hpp
/usr/include/boost/variant/detail/cast_storage.hpp
/usr/include/boost/variant/detail/config.hpp
/usr/include/boost/variant/detail/element_index.hpp
/usr/include/boost/variant/detail/enable_recursive.hpp
/usr/include/boost/variant/detail/enable_recursive_fwd.hpp
/usr/include/boost/variant/detail/forced_return.hpp
/usr/include/boost/variant/detail/generic_result_type.hpp
/usr/include/boost/variant/detail/has_result_type.hpp
/usr/include/boost/variant/detail/hash_variant.hpp
/usr/include/boost/variant/detail/initializer.hpp
/usr/include/boost/variant/detail/make_variant_list.hpp
/usr/include/boost/variant/detail/move.hpp
/usr/include/boost/variant/detail/multivisitors_cpp11_based.hpp
/usr/include/boost/variant/detail/multivisitors_cpp14_based.hpp
/usr/include/boost/variant/detail/multivisitors_preprocessor_based.hpp
/usr/include/boost/variant/detail/over_sequence.hpp
/usr/include/boost/variant/detail/substitute.hpp
/usr/include/boost/variant/detail/substitute_fwd.hpp
/usr/include/boost/variant/detail/variant_io.hpp
/usr/include/boost/variant/detail/visitation_impl.hpp