With --copy-dest, rsync tries to copy basis files with unchanged data into the destination even in --dry-run mode; fortunately, the attempt is blocked by do_open.  Rsync should itemize the local copy without trying to actually perform it.

To reproduce this bug:

mkdir src dest basis
echo foo >src/file
echo foo >basis/file
rsync -r -ni --checksum --copy-dest=../basis/ src/ dest/

Result:

rsync: open "/path/to/dest/file": Success (0)
>f+++++++++ file
rsync error: some files could not be transferred (code 23) at main.c(1053) [sender=3.0.0pre4]