:= works with GForce

[ben-schwen]

Closes #1414

[codecov]

Codecov Report

Merging #5245 (b815f54) into master (9619ad5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5245   +/-   ##
=======================================
  Coverage   99.50%   99.50%           
=======================================
  Files          77       77           
  Lines       14647    14655    +8     
=======================================
+ Hits        14575    14583    +8     
  Misses         72       72           

Impacted Files	Coverage Δ
R/data.table.R	99.89% <100.00%> (+<0.01%)	⬆️

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[mattdowle]

Failed GLCI in R 3.5.0, 3.4.4 and 3.1.0 due to the following, using test 1085 as example and R 3.1.0 :

Browse[2]> ans
[[1]]
[1] 1 2

[[2]]
[1] "1997-06-02" "1998-01-11"

Browse[2]> sapply(ans,class)
[[1]]
[1] "integer"

[[2]]
[1] "IDate" "Date" 

Browse[2]> length(g)
[1] 1
Browse[2]> len__
[1] 5 5
Browse[2]> class(len__)
[1] "integer"
Browse[2]> lapply(tail(ans, -length(g)), rep.int, times = len__)   # in this PR as of now
[[1]]
 [1] 10014 10014 10014 10014 10014 10237 10237 10237 10237 10237

Browse[2]> lapply(tail(ans, -length(g)), rep, times = len__)  # changing `rep.int` to `rep` appears to work
[[1]]
 [1] "1997-06-02" "1997-06-02" "1997-06-02" "1997-06-02" "1997-06-02"
 [6] "1998-01-11" "1998-01-11" "1998-01-11" "1998-01-11" "1998-01-11"

so I'll change rep.int to rep to pass. I don't know in which version of R after 3.5.0 that this expression started to retain the class. In R 3.6.0 there was a news item "as.call(), rep.int(), rep_len() and nchar() dispatch internally.` and in R 4.0.0 there was "When internal dispatch for rep.int() and rep_len() fails, there is an attempt to dispatch on the equivalent call to rep()."