Cleanup MissingType enum constants

[AlbertoEAF]

• Clean up comparisons of missing_type using the enum name instead of hard-coded constants

  This does not incur in any overhead and eases code comprehension.

[AlbertoEAF]

P.S.: I placed this PR whilst trying to understand the code in #2921.

[AlbertoEAF]

@guolinke, do you approve this refactor? :)

[guolinke]

LGTM

[jameslamb]

If you rebase to master, to get the changes from #2954 , the R test issues and linting issues should be resolved. That will also fix any weirdness in the Travis build, which was a result of the recent Travis outage.

Sorry for the inconvenience!

[AlbertoEAF]

Done @jameslamb @guolinke :)

[jameslamb]

Done @jameslamb @guolinke :)

Thank you! The Travis failures left (on check-docs) task are unrelated to this PR and being fixed in #2956 . @guolinke if you are ok with the current state of this PR (it's been a few days since your approval), you can use your admin power to merge this despite failing CI since all other checks except check-docs are passing.

[StrikerRUS]

The Travis failures left (on check-docs) task are unrelated to this PR and being fixed in #2956 .

Merged now.

[AlbertoEAF]

I think it was approved but not merged @StrikerRUS ?

[jameslamb]

I think it was approved but not merged @StrikerRUS ?

it was for sure merged

[AlbertoEAF]

Sorry, I misunderstood, thought you were talking about this merge, not #2956 :)

[AlbertoEAF]

Can this be approved @guolinke ?

[StrikerRUS]

@AlbertoEAF Please rebase to master. It will allow us to merge this PR.

[AlbertoEAF]

Done @StrikerRUS :)

[StrikerRUS]

@AlbertoEAF Thank you very much for your help!

[github-actions]

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