I didn't realize DT[TRUE] was a way to achieve a shallow copy. shallow copy was only intended for internal use. Thanks to @renkun-ken for highlighting this in #3214, and related #2254.

• dt[TRUE] no longer performs shallow copy #3214
• Inconsistent copy behavior when subsetting #2254

In v1.11.8 we see this :

DT = data.table(id = 1:5, key="id")
DT1 = DT[TRUE]
key(DT1)
[1] "id"
DT1[3, id:=6L]
key(DT1)
# NULL              # correct
DT$id
# [1] 1 2 6 4 5     # should be 1:5
key(DT)
# [1] "id"          # invalid key

It only occurs after DT[TRUE], iiuc, which hopefully folk have not discovered or relied on too much?! I hope the usage out there is like @renkun-ken described to add new columns to the shallow copy, not to change existing columns!

New test 1542.08 was added in PR #2313 ready for when this is fixed.