table retain's SD's locked attribute outside [

```
x <- data.table(a = rnorm(3), b = rnorm(3))
x <- x[, round(.SD, 1)]
x[, c := 8] 
# Error in `[.data.table`(x, , `:=`(c, 8)) :
# .SD is locked. Using := in .SD's j is reserved for possible future use; ...
```