Inconsistent behaviour of assign := operator when grouping or not

[pietrodito]

`# Minimal example

library(data.table)

DT <- data.table(col = c('a', 'a_b', 'a_b_c'))

DT[, c('col1', 'col2') := tstrsplit(col, '_')]
## Produces error: 
## Error in `[.data.table`(DT, , `:=`(c("col1", "col2"), tstrsplit(col, "_"))) : 
## Supplied 2 columns to be assigned 3 items. Please see NEWS for v1.12.2.

DT <- data.table(col = c('a', 'a_b', 'a_b_c'))

DT[, c('col1', 'col2') := tstrsplit(col, '_'), by = .(col)]
DT
## works:
##      col col1 col2
## 1:     a    a    a
## 2:   a_b    a    b
## 3: a_b_c    a    b

I am surprised that the second code does not produce an error.

---

The same confusion happens when one does that:

DT1 <- data.table(col1 = c('a', 'a_b'))
DT2 <- copy(DT1)

DT1[, c('col_a', 'col_b') := tstrsplit(col1, '_')]
DT2[, c('col_a', 'col_b') := tstrsplit(col1, '_'), by = .(col1)]

DT1
#    col1 col_a col_b
# 1:    a     a  <NA>
# 2:  a_b     a     b
DT2
#    col1 col_a col_b
# 1:    a     a     a
# 2:  a_b     a     b

# Output of sessionInfo()

R version 4.0.4 (2021-02-15)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 19041)

Matrix products: default

locale:
[1] LC_COLLATE=French_France.1252  LC_CTYPE=French_France.1252   
[3] LC_MONETARY=French_France.1252 LC_NUMERIC=C                  
[5] LC_TIME=French_France.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.14.0

loaded via a namespace (and not attached):
[1] compiler_4.0.4 tools_4.0.4

[mt1022]

Here are the links to the related SO posts:
https://stackoverflow.com/q/67091535/3926543
https://stackoverflow.com/q/67090605/3926543

[ben-schwen]

Might be a duplicate of #4022 or #3256