/usr/include/d2/4.6/std/c/ is in libphobos2-4.6-dev 0.29.1-4.6.3-1ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/d2/4.6/std/c/fenv.d
/usr/include/d2/4.6/std/c/linux/
/usr/include/d2/4.6/std/c/linux/linux.d
/usr/include/d2/4.6/std/c/linux/pthread.d
/usr/include/d2/4.6/std/c/linux/socket.d
/usr/include/d2/4.6/std/c/linux/termios.d
/usr/include/d2/4.6/std/c/linux/tipc.d
/usr/include/d2/4.6/std/c/locale.d
/usr/include/d2/4.6/std/c/math.d
/usr/include/d2/4.6/std/c/osx/
/usr/include/d2/4.6/std/c/osx/socket.d
/usr/include/d2/4.6/std/c/process.d
/usr/include/d2/4.6/std/c/stdarg.d
/usr/include/d2/4.6/std/c/stddef.d
/usr/include/d2/4.6/std/c/stdio.d
/usr/include/d2/4.6/std/c/stdlib.d
/usr/include/d2/4.6/std/c/string.d
/usr/include/d2/4.6/std/c/time.d
/usr/include/d2/4.6/std/c/unix/
/usr/include/d2/4.6/std/c/unix/unix.d
/usr/include/d2/4.6/std/c/wcharh.d
/usr/include/d2/4.6/std/c/windows/
/usr/include/d2/4.6/std/c/windows/com.d
/usr/include/d2/4.6/std/c/windows/stat.d
/usr/include/d2/4.6/std/c/windows/windows.d
/usr/include/d2/4.6/std/c/windows/winsock.d