fix test; add todo

Author: MichaelChirico

R/data.table.R

@@ -2568,6 +2568,7 @@ split.data.table = function(x, f, drop = FALSE, by, sorted = FALSE, keep.by = TR
   if (flatten || length(by) == 1L) {
     for (xi in ll) .Call(C_unlock, xi)
     out = lapply(ll, setDT)
+    # TODO(#2000): just let setDT handle this
     if (!identical(old_class <- class(x), c("data.table", "data.frame"))) for (xi in out) setattr(xi, "class", old_class)
     out
     # alloc.col could handle DT in list as done in: c9c4ff80bdd4c600b0c4eff23b207d53677176bd

inst/tests/tests.Rraw

@@ -9766,7 +9766,7 @@ test(1639.143, x = split(dt, by = "y"), y = list(`1` = dt))
 
 # retain a custom class after splitting, #7105
 DT = data.table(x=letters[1:10], y=1:10, z=rnorm(10))
-setattr(DT, "class", c("my_class", class(dt)))
+setattr(DT, "class", c("my_class", class(DT)))
 test(1639.144, "my_class" %in% unlist(lapply(split(DT, by="x"), class)))
 test(1639.145, "my_class" %in% unlist(lapply(split(DT, ~x), class)))
 test(1639.146, "my_class" %in% unlist(lapply(split(DT, by=c("x", "y")), class)))