Failed to extract grouped list values

[hongyuanjia]

Related to #4758 and hongyuanjia/eplusr#346

# Current GitHub version
dt <- data.table(
    value = as.list(1:6),
    index = rep(1:2, each = 3)
)

dt[, by = "index", list(value = list(value))]$value
# [[1]]
# [[1]][[1]]
# [1] 4
# 
# [[1]][[2]]
# [1] 5
# 
# [[1]][[3]]
# [1] 6
# 
# 
# [[2]]
# [[2]][[1]]
# [1] 4
# 
# [[2]][[2]]
# [1] 5
# 
# [[2]][[3]]
# [1] 6

# Current CRAN version
dt <- data.table::data.table(
    value = as.list(1:6),
    index = rep(1:2, each = 3)
)

dt[, by = "index", list(value = list(value))]$value
# [[1]]
# [[1]][[1]]
# [1] 1
# 
# [[1]][[2]]
# [1] 2
# 
# [[1]][[3]]
# [1] 3
# 
# 
# [[2]]
# [[2]][[1]]
# [1] 4
# 
# [[2]][[2]]
# [1] 5
# 
# [[2]][[3]]
# [1] 6

[ben-schwen]

Even shorter example:

library(data.table)
data.table(value = as.list(1:2), index = 1:2)[,.(value = list(value)), index][["value"]]

This was introduced by commit 9396741 (#4655)