Fix Rdatatable#4981: don't trust all.vars when its arguments contains…

… get/eval and siblings.

R/data.table.R

@@ -756,7 +756,14 @@ replace_dot_alias = function(e) {
  bysub = parse(text=paste0("list(",paste(bysub,collapse=","),")"))[[1L]]
  bysubl = as.list.default(bysub)
  }
- allbyvars = intersect(all.vars(bysub), names_x)
+ # Fix 4981: when the 'by' expression includes get/mget/eval, all.vars
+ # cannot be trusted to infer all used columns
+ bysub.elems <- rapply(as.list(bysub), as.character)
+ if (any(c("eval","evalq","eval.parent","local","get","mget","dynGet") %chin% bysub.elems)) 
+ allbyvars = NULL
+ else
+ allbyvars = intersect(all.vars(bysub), names_x) 
+
  orderedirows = .Call(CisOrderedSubset, irows, nrow(x)) # TRUE when irows is NULL (i.e. no i clause). Similar but better than is.sorted(f__)
  bysameorder = byindex = FALSE
  if (!bysub %iscall% ":" && ##Fix #4285