Fixed bug in mean -> fastmean during #735. Added test.

R/data.table.R

@@ -1383,10 +1383,11 @@ chmatch2 <- function(x, table, nomatch=NA_integer_) {
             nomeanopt=FALSE  # to be set by .optmean() using <<- inside it
             oldjsub = jsub
             if (jsub[[1L]]=="list") {
-                for (ii in seq_along(jsub)[-1L])
+                for (ii in seq_along(jsub)[-1L]) {
                     if (dotN(jsub[[ii]])) next; # For #5760
                     if (is.call(jsub[[ii]]) && jsub[[ii]][[1L]]=="mean")
                         jsub[[ii]] = .optmean(jsub[[ii]])
+                }
             } else if (jsub[[1L]]=="mean") {
                 jsub = .optmean(jsub)
             }

inst/tests/tests.Rraw

@@ -4907,6 +4907,9 @@ test(1362.13, DT[, head(.SD, 1), by=x, verbose=TRUE],
 test(1362.14, names(DT[, c(list(.I, mean(y)), lapply(.SD, sum)), by=x]), c("x", "I", "V2", "y", "z"))
 # and if a name is given, it's retained
 test(1362.15, names(DT[, c(list(bla=.I, mean(y)), lapply(.SD, sum)), by=x]), c("x", "bla", "V2", "y", "z"))
+# Add test to ensure that mean() gets replaced with fastmean when GForce won't be used.
+test(1362.16, DT[, c(list(.I, mean(y)), lapply(.SD, mean)), by=x, verbose=TRUE], 
+  output="Old mean optimization changed j from 'list(.I, mean(y), mean(y), mean(z))' to 'list(.I, .External(Cfastmean, y, FALSE), .External(Cfastmean, y, FALSE), .External(Cfastmean, z, FALSE))'")
 
 # setDT(DT), when input is already a data.table checks if selfrefok and if not, does alloc.col again.
 DT = list(data.frame(x=1:5, y=6:10))