applied suggestion from Kurt Hornik

[mattdowle]

Matt,

Recent r-devel improvements showed that in merge.data.table you relied
on

R> intersect("A", NULL)
NULL

which was more of an implementation "detail" than really intended, with
the old implementation also giving

R> intersect(NULL, "A")
character(0)

I have (for now) changed intersect() to consistently give NULL when one
of the operands is NULL, so merge.data.table works "as intended", but
perhaps you want to

    if (is.null(by))
        by = intersect(key(x), key(y))
    if (is.null(by))
        by = key(x)
    if (is.null(by))
        by = intersect(names(x), names(y))

to something like

    if (is.null(by))
        by = intersect(key(x), key(y))
    if (!length(by))
        by = key(x)
    if (!length(by))
        by = intersect(names(x), names(y))

to be on the safe side?

Best
-k

[codecov]

Codecov Report

Merging #5183 (b77137e) into master (bbe4164) will not change coverage.
The diff coverage is 100.00%.

❗ Current head b77137e differs from pull request most recent head 063b883. Consider uploading reports for the commit 063b883 to get more accurate results

@@           Coverage Diff           @@
##           master    #5183   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          77       77           
  Lines       14540    14540           
=======================================
  Hits        14451    14451           
  Misses         89       89           

Impacted Files	Coverage Δ
R/merge.R	100.00% <100.00%> (ø)

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