Fix warning on Windows.

[trivialfis]

Targeted at 1.4 branch, will port to master once merged. Address warning found by @hetong007 in #6793 (comment) .

Do not merge until all tests on R-hub have passed. (Huge thanks to R-hub). Progress can be tracked here:

• macOS 10.13.6 High Sierra, R-release, CRAN's setup
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-0c3c8a0f078d60e5fb1d1b19430036d9

• Windows Server 2008 R2 SP1, R-release, 32/64 bit
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-bc2de41bceddd51493632f62f3434c78

• Oracle Solaris 10, x86, 32 bit, R-release
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-66c959222fbaf97afac3f721205f9ae4

• Oracle Solaris 10, x86, 32 bit, R-release, Oracle Developer Studio 12.6
  Failing due to compiler couldn't infer dispatching type.
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-bdf750d127a041e6fcc77bba0017480c

• Debian Linux, R-release, GCC
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-bc0161ccee0b610ce564d0f2b51892e4

• CentOS 8, stock R from EPEL
  Failing in dependency compilation. boost required by readr is failing to compile.
  https://builder.r-hub.io/status/xgboost_1.4.1.1.tar.gz-c2a2446e658c69a1891508cb3a7afbf4

[trivialfis]

@hetong007 Hi, I ran some tests as listed in the PR description. There are 2 failures, one is on Solaris + Oracle Developer Studio 12.6, another one is due to the indirect dependency on readr (PREPERROR). Others are passing.

• I think the readr should be fine on cran based on its current check status: https://cran.r-project.org/web/checks/check_results_readr.html .
• For the issue with Solaris + Oracle Developer Studio 12.6, I'm not sure if it's necessary for us to fix it. Working around that compiler is not trivial. ;-(

[codecov-commenter]

Codecov Report

Merging #6883 (c7b94e6) into release_1.4.0 (a347ef7) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           release_1.4.0    #6883   +/-   ##
==============================================
  Coverage          81.80%   81.80%           
==============================================
  Files                 13       13           
  Lines               3852     3852           
==============================================
  Hits                3151     3151           
  Misses               701      701           

---

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[hetong007]

@trivialfis Thanks for the efforts! I think the Solaris + Oracle Developer Studio 12.6 environment is challenging, and based on my experiences from the last submission, errors/warnings on this environment is not the same as the CRAN machine, thus I'd say it could be just fine. And the readr dependency should be fine as well, exactly as you mentioned.

I'm seeing some CI failures, do you suggest me to wait for an update, or should I just apply the current patch and re-submit?

[Craigacp]

I can't see the Solaris error, when I went to those links it looked like everything worked (but they were both using GCC 5), but I've got experience working on Solaris 11 with Developer Studio 12.6 so I might be able to take a look at some point. We had XGBoost running on Solaris 11 SPARC several years ago.

Is there a link to the actual failure somewhere?

[trivialfis]

@hetong007 I re-ran the test, the previous failure seems to be internal to GitHub action.

The test on centos 8 seems to be related to LaTeX output, I got confused by the long expanded macro from the boost library.

12845#> Loading required package: knitr
12846#> Error: processing vignette 'xgboost.Rnw' failed with diagnostics:
12847#> Running 'texi2dvi' on 'xgboost.tex' failed.
12848#> LaTeX errors:
12849#> ! LaTeX Error: File `framed.sty' not found.
12850#> Type X to quit or to proceed,
12851#> or enter new name. (Default extension: sty)
12852#> ! Emergency stop.

But I think cran should have a working TeX distribution.

@Craigacp Sorry those are the public links from R-hub. I receive email notification for each run, here are the artifacts containing build logs:

• Failure on Solaris due to template deduction error: https://artifacts.r-hub.io/xgboost_1.4.1.1.tar.gz-bdf750d127a041e6fcc77bba0017480c/xgboost.Rcheck/00install.out

[trivialfis]

@hetong007 I will merge this PR into the 1.4 branch. Feel free to fetch from this branch.

Feel free to ping me in #6793 if there's anything else I can help.