Warning for set / := when numToDo > nrow(x)?

Follow-up to #2837 inspired by #1885 / #3460

We decided running duplicated every time we run set/:= to help protect against duplicated assignment is overkill & an efficiency killer.

However, when length(i) > nrow(x), there's no need to run duplicated -- there's guaranteed to be at least one duplicate in i (Pigeonhole):

DT = data.table(a = 1L)
DT[c(1, 1), a := 2:3]

We could warn in this case?

unless DT[c(1, NA), a := 2:3]

@jangorecki

Re @jangorecki 's example, you can use numToDo instead of length(i) then? That count is already recorded with NAs removed as seen in

> DT[c(1, NA), a := 2:3, verbose=TRUE]
Detected that j uses these columns: a 
Assigning to 1 row subset of 1 rows
RHS_list_of_columns == false

I guess this would just be adding a warning near the message above

Line 344 in 2385354

if (verbose) Rprintf("Assigning to %d row subset of %d rows\n", numToDo, nrow);

Related #2022

Thanks Frank/Jan, yes length(i) is not the condition we should use but it seems numToDo would be the corrected adjustment.

Main Q is whether we thing this is warning-worthy behavior? Or perhaps just something to append to/reiterate in the verbose message?

@MichaelChirico

@MichaelChirico lets start with verbose message, this at least won't make any breaking changes

isn't it duplicate of #2022 ?

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