[datatable-help] Problem with keys when by= generates NAs

[Muhammad Waliji]

The following warning occurs in both versions 1.6.6 and 1.7.1.  I haven't
tried other versions.

> DT <- data.table(var1=rep(c(1, 2), each=5),
+                  var2=seq(0, 1, length.out=10),
+                  key="var1")
> DT.2 <- DT[, list(sum(var2)), list(ifelse(var1 == 1, 1, NA))]
> setkey(DT.2, var1)
Warning message:
In setkey(DT.2, var1) :
  Already keyed by this key but had invalid row order, key rebuilt. If you
didn't go under the hood please let maintainer('data.table') know so the
root cause can be fixed.
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