colnamesInt() has unfriendly behavior w.r.t. missing (NA) columns

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
r
Domain
data

Research direction

Start at the private colnamesInt() entry point and inspect its call sites and existing tests to determine whether missing inputs can reach it. If NA is possible, define and test friendlier behavior; otherwise remove tests covering impossible missing inputs. Verify the relevant data.table tests after making the chosen change.

Written by the indexing model from the issue text.

Description

consistency internals
          It's a bit strange that `NA` returns `0L` for character input, but `NA_integer_` for numeric input.

In fact I find the current behavior odd as well:

data.table:::colnamesInt(iris, NA_integer_)
# Error in data.table:::colnamesInt(iris, NA_integer_) : 
#   argument specifying columns received non-existing column(s): cols[1]=-2147483648

I think it's outside the scope of this PR though, I'll file a follow-up issue.

Originally posted by @MichaelChirico in https://github.com/Rdatatable/data.table/pull/6068#discussion_r1560148562

First, we should check that it's actually possible to provide NA to this column since it's a private function (maybe we fully control all inputs & know for sure it can't receive missing inputs).

If user might provide NA, we should handle this better.

If NA input is impossible, we should drop tests with missing input.

Dominant language
R
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