/usr/include/spdlog/details/ is in libspdlog-dev 1.6-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/spdlog/details/async_log_helper.h
/usr/include/spdlog/details/async_logger_impl.h
/usr/include/spdlog/details/file_helper.h
/usr/include/spdlog/details/format.cc
/usr/include/spdlog/details/format.h
/usr/include/spdlog/details/line_logger.h
/usr/include/spdlog/details/log_msg.h
/usr/include/spdlog/details/logger_impl.h
/usr/include/spdlog/details/mpmc_bounded_q.h
/usr/include/spdlog/details/null_mutex.h
/usr/include/spdlog/details/os.h
/usr/include/spdlog/details/pattern_formatter_impl.h
/usr/include/spdlog/details/registry.h
/usr/include/spdlog/details/spdlog_impl.h