DT = data.table(i = c(1L, 1L), x = lim.integer64()[2L])

DT[, sum(x), by=i]
#        i    V1
#    <int> <i64>
# 1:     1    -2
DT[, base::sum(x), by=i]
#        i    V1
#    <int> <i64>
# 1:     1  <NA>
# Warning message:
# In sum.integer64(c(NaN, NaN), na.rm = FALSE) :
#   NAs produced by integer64 overflow