Linux has similar behaviour about d_ino, returning the inode number from the parent filesystem rather than the mount point itself.  This is a POSIX violation, but there's little interest in fixing it: https://groups.google.com/forum/#!topic/linux.kernel/4rzZRWKWT9c  For similar reasons, I expect this behaviour won't change in the Linux kernel :(

So what to do?  It's quite the pessimization to require stat() for -type.  (Technically -type d can be answered without stat() because directories can only be mounted over directories.)  I think trusting d_type for everything except mount points is a good idea.

yes, getdents returns d_type=DT_REG, while the later newfstatat - needed for the -ls action - returns st_mode=S_IFCHR:

getdents(4, [
  {d_ino=319425, d_off=1, d_reclen=24, d_name=".", d_type=DT_DIR},
  {d_ino=13363,  d_off=2, d_reclen=24, d_name="..", d_type=DT_DIR},
  {d_ino=322564, d_off=3, d_reclen=40, d_name="bindmntoverfile", d_type=DT_REG},
  {d_ino=319462, d_off=4, d_reclen=24, d_name="null", d_type=DT_CHR}], 32768) = 112
...
newfstatat(5, "bindmntoverfile", {
  st_dev=makedev(0, 21),
  st_ino=319462,
  st_mode=S_IFCHR|0644,
  st_nlink=1,
  st_uid=0,
  st_gid=0,
  st_blksize=4096,
  st_blocks=0,
  st_rdev=makedev(1, 3),
  st_atime=1540884672 /* 2018-10-30T08:31:12.131190660+0100 */, st_atime_nsec=131190660,
  st_mtime=1540884672 /* 2018-10-30T08:31:12.131190660+0100 */, st_mtime_nsec=131190660,
  st_ctime=1540884672 /* 2018-10-30T08:31:12.131190660+0100 */, st_ctime_nsec=131190660},
  AT_SYMLINK_NOFOLLOW) = 0

At least this is not an FTS regression, as 'oldfind' suffered from the same issue.

Nice catch - and a surprising use case of bind mounts!

And it's interesting that the output of the -ls action shows
the correct type again - of course, because that needs to do an
additional stat() on the file.

FWIW: this issue happens with all non-directory bind-mounts
on top of another type, e.g.
- socket -> regfile,
- regfile -> socket,
- character file -> socket
and so on.

I'll have to check if the change is done on the right place;
similar adjustment based on ent->ftsinfo are also in consider_visiting().

Furthermore, I'd like to get the coreutils-style test-framework
from gnulib into findutils, so that we could also have tests
like for this issue which require 'root' permissions.