No realloc warning when adding many inside a function

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
55/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Quiet
Tech stack
r
Domain
data

Research direction

Reproduce the reported examples with data.table 1.17.99, comparing := in a function with set(). Trace the allocation path that should emit the realloc warning, then add a regression test covering the missing warning and confirm the reported dimension behavior is handled.

Written by the indexing model from the issue text.

Description

bug 
packageVersion("data.table")
# [1] ‘1.17.99’
x = data.table(a=1:3)
ncol = truelength(x)
ncol
# [1] 1025

f = function(DT) {
 for (i in seq_len(ncol + 100L)) {
   DT[,paste0("new",i):=1:3]
 }
}
f(x)            # I thought there was a warning here that := on i==ncol did a realloc
dim(x)
# [1]   3 1025       # wrong result

f = function(DT) {
  DT[, paste0("new", seq_len(ncol + 100L)) := 1:3]
  NULL
}
f(x)
dim(x)
# [1]   3 1025       # wrong result

f = function(DT) {
 for (i in seq_len(ncol + 100L)) {
   DT[,paste0("new",i):=1:3]
 }
 DT                            # needed in this case
}
x = f(x)                       # x= needed in this case
dim(x)
# [1]   3 1126                      # correct result

x = data.table(a = 1:3)
f = function(DT) {
  for (j in seq_len(ncol + 100L)) set(DT, NULL, paste0("new", j), 1:3)
}
f(x)
# Error in set(DT, NULL, paste0("new", j), 1:3) : 
#   Internal error in assign: input dt has not been allocated enough column slots. l=1025, tl=1025, adding 1. Please report to the data.table issues tracker.
# dim(x)

I seem to remember there was a warning. Maybe it isn't happening in this case for some reason. Need to reinstate the warning and add a test for it.

Dominant language
R
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