.SDcols is a negation only for _unary_ '-' (#5881)

* fix issue with .SDcols using binary "-"

NEWS.md

@@ -341,7 +341,7 @@
 
 23. `fread(fill=TRUE, verbose=TRUE)` would segfault on the out-of-sample type bump verbose output if the input did not contain column names, [5046](https://github.com/Rdatatable/data.table/pull/5046). Thanks to Václav Tlapák for the PR.
 
-24. `.SDcols=-V2:-V1` and `.SDcols=(-1)` could error with `xcolAns does not pass checks` and `argument specifying columns specify non existing column(s)`, [#4231](https://github.com/Rdatatable/data.table/issues/4231). Thanks to Jan Gorecki for reporting and the PR.
+24. `.SDcols=-V2:-V1` and `.SDcols=(-1)` could error with `xcolAns does not pass checks` and `argument specifying columns specify non existing column(s)`, [#4231](https://github.com/Rdatatable/data.table/issues/4231). Thanks to Jan Gorecki for reporting and the PR. Thanks Toby Dylan Hocking for tracking down an error caused by the initial fix and Michael Chirico for fixing it.
 
 25. `.SDcols=<logical vector>` is now documented in `?data.table` and it is now an error if the logical vector's length is not equal to the number of columns (consistent with `data.table`'s no-recycling policy; see new feature 1 in v1.12.2 Apr 2019), [#4115](https://github.com/Rdatatable/data.table/issues/4115). Thanks to @Henrik-P for reporting and Jan Gorecki for the PR.
 

R/data.table.R

@@ -1008,7 +1008,8 @@ replace_dot_alias = function(e) {
           # peel from parentheses before negation so (-1L) works as well: as.data.table(as.list(1:3))[, .SD,.SDcols=(-1L)] #4231
           while(colsub %iscall% "(") colsub = as.list(colsub)[[-1L]]
           # fix for R-Forge #5190. colsub[[1L]] gave error when it's a symbol.
-          if (colsub %iscall% c("!", "-")) {
+          # NB: _unary_ '-', not _binary_ '-' (#5826). Test for '!' length-2 should be redundant but low-cost & keeps code concise.
+          if (colsub %iscall% c("!", "-") && length(colsub) == 2L) {
             negate_sdcols = TRUE
             colsub = colsub[[2L]]
           } else negate_sdcols = FALSE

inst/tests/tests.Rraw

@@ -18179,3 +18179,8 @@ DT = data.table(id=1:2, x=1:2)
 r = copy(DT)[1L, x:= 5L]
 test(2241.13, DT, data.table(id=1:2, x=1:2))
 test(2241.14, r, data.table(id=1:2, x=c(5L,2L)))
+
+# .SDcols using binary '-' is evaluated correctly (#5826 exposed this)
+DT = data.table(a=1, b=2, c=3)
+test(2242.1, DT[, .SD, .SDcols=2-1], DT[, .(a)])
+test(2242.2, DT[, .SD, .SDcols=length(DT)-1], DT[, .SD, .SDcols=2])