fread(integer64 = "double") not working for some data

Start with the reported fread("test_data.txt", sep = "|", integer64 = "double") reproducer and compare its behavior with the release version. Trace the integer64 handling used by fread and add a regression test using the attached test_data.txt. Done means the affected column is read as a numeric/double column when integer64 is set to "double".

I'm testing the latest development version of data.table and find that fread does not respect integer64 = "double" for some of my data. There's no such problem in the release version.

The test code is:

dt <- fread("test_data.txt", sep = "|", integer64 = "double")

but the resulted data.table still has integer64 column:

> str(dt)
lasses ‘data.table’ and 'data.frame':	2583 obs. of  15 variables:
 $ V1 : num  0.1 0.01 0.04 0.04 0.02 NA 0.01 0.01 0.03 0.02 ...
 $ V2 : num  0.0509 0.01 0.0138 0.0248 0.0141 ...
 $ V3 : num  0.0451 0.01 0.0153 0.0445 0.0133 ...
 $ V4 : num  0.0386 0.01 0.0153 0.0557 0.0129 ...
 $ V5 :integer64 1020396 55949051 4935942 3668540 11818540 30119787 115742884 0 ... 
 $ V6 : num  1734596 60721591 10311588 1711172 15439786 ...
 $ V7 : num  1541020 46302203 5275696 1276379 13350567 ...
 $ V8 : num  1408261 42321135 3844999 1173221 11545387 ...
 $ V9 : num  9282004 84280297 15006800 14062030 81537656 ...
 $ V10: logi  NA NA NA NA NA NA ...
 $ V11: logi  NA NA NA NA NA NA ...
 $ V12: logi  NA NA NA NA NA NA ...
 $ V13: logi  NA NA NA NA NA NA ...
 $ V14: logi  NA NA NA NA NA NA ...
 $ V15: num  2.35e+09 2.45e+09 1.11e+09 2.28e+09 5.38e+09 ...
 - attr(*, ".internal.selfref")=<externalptr> 

The data is attached below:

test_data.txt

The same happens on both macOS and Ubuntu as I tested.

Here's my session info:

R version 3.4.3 (2017-11-30)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.3

Matrix products: default
BLAS: /System/Library/Frameworks/Accelerate.framework/Versions/A/Frameworks/vecLib.framework/Versions/A/libBLAS.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/3.4/Resources/lib/libRlapack.dylib

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.10.5

loaded via a namespace (and not attached):
 [1] bit_1.1-12      httr_1.3.1      compiler_3.4.3  R6_2.2.2        tools_3.4.3     withr_2.1.1     curl_3.1        yaml_2.1.16    
 [9] memoise_1.1.0   bit64_0.9-7     knitr_1.19      git2r_0.21.0    digest_0.6.15   devtools_1.13.4

R version 3.4.3 (2017-11-30)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 16.04.3 LTS

Matrix products: default
BLAS: /usr/lib/openblas-base/libblas.so.3
LAPACK: /usr/lib/libopenblasp-r0.2.18.so

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C               LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.10.5

loaded via a namespace (and not attached):
[1] compiler_3.4.3 tools_3.4.3    yaml_2.1.16