This ... depends. In normal POSIX land, hard links break the tree structure, for one, so you get a DAG but not a tree. I think some file systems do enforce tree structure, though - hard links are not supported everywhere.

It used to be possible ages ago to hard link to directories, which meant that you could have actual cycles and a recursive tree-walking algorithm would never terminated. (As far as I know you can still do this by editing the disk manually, although I think fsck will make a fuss if it detects this.)

You can still, with the right syscalls and drivers, do something like hard links on NTFS (I think they're technically called mount points but it's not the same thing as POSIX ones). I'm not sure if you can still make directory cycles, and you're probably a bad person if you do.