I'm asking because I'm trying to identify the root cause(s) for the warnings about "Invalid .internal.selfref detected and fixed" that I have triggered on five occasions while developing code using data.table. It seems pretty clear that, somewhere along the line, my code performs an operation on a data.table which causes it to be copied -- at least under some conditions. It's a long path to the warning, and my code-inspection hasn't revealed any likely suspects. Until I can localise the defect, I can't produce an MRE -- nor can I be confident of the integrity of the data.tables produced by my code.
A reasonably-efficient way to localise what I'm tentatively labelling an "inadvertent copy defect" would be to pepper-pot my code with stopifnot() invocations. But! I can't figure out how to write a valid.internal.selfref() method.
Section 5.13 of Writing R Extensions tells me that this method can't be written in R. I'm pretty sure there'll be a C-language internal method of data.table which guards the "Invalid .internal.selfref detected and fixed" warning.
Warning: 'Invalid .internal.selfref detected' when adding a column to a data.table returned from a function has a very nice explanation of .internal.selfref, but fails to reveal (at least in my reading) how a user of data.table could test this sentinel.
Looking at data.table source code, this warning seems to be triggered here.
This means you could use the internal data.table:::selfrefok function:
library(data.table)
data.table:::selfrefok(data.table(x=1))
#> [1] 1
data.table:::selfrefok(data.frame(x=1))
#> [1] 0