Enable coverage and docs

[DieBauer]

Addresses two points of #2347

Enable scoverage for 2.13 and enable scaladoc generation for 2.13.

One caveat with scoverage is that, for scala 2.13.1 (which Cats currently doesn't use), there are issues as described here scoverage/sbt-scoverage#295 and blocking an upgrade.

This might be decisive in dropping that commit from this PR, let me know.

[codecov-io]

Codecov Report

Merging #3096 into master will decrease coverage by 0.35%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3096      +/-   ##
==========================================
- Coverage   93.52%   93.17%   -0.36%     
==========================================
  Files         368      372       +4     
  Lines        6998     7179     +181     
  Branches      198      207       +9     
==========================================
+ Hits         6545     6689     +144     
- Misses        453      490      +37

Flag	Coverage Δ
#scala_version_212	93.52% <ø> (?)
#scala_version_213	90.86% <ø> (?)

Impacted Files	Coverage Δ
...e/src/main/scala-2.13+/cats/data/ZipLazyList.scala	77.77% <0%> (ø)
...src/main/scala-2.13+/cats/instances/lazyList.scala	100% <0%> (ø)
....13+/cats/kernel/instances/LazyListInstances.scala	100% <0%> (ø)
.../main/scala-2.13+/cats/data/NonEmptyLazyList.scala	60.86% <0%> (ø)
...in/scala/cats/data/AbstractNonEmptyInstances.scala	100% <0%> (+3.44%)	⬆️

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[kailuowang]

As of now on travis we only run coverage on Scala 2.12 using this job

With this change we shall be able to delete this coverage dedicated job and run coverage for both 2.12 and 2.13 by changing the jvm tests job to do coverage as well. I.e. change the following
https://github.com/typelevel/cats/blob/master/.travis.yml#L47
to

      install: pip install --user codecov
      script: sbt ++$TRAVIS_SCALA_VERSION!  coverage buildJVM bench/test coverageReport && codecov

[DieBauer]

@kailuowang thanks for the remark, I forgot about travis.

I'm reading the PR (#2737) that added the 2.12 coverage job, and there it states that running coverage reports for multiple scala versions is 'undefined'.

So I'm not entirely sure if this is what we want to enable here. What are your thoughts?

Also, regarding publishing API docs for 2.12 and 2.13 does anything need to change? I'm not familiar with microsite. In my projects I'm doing +unidoc and move the /target/docs/scala-x to a server.

[kailuowang]

My thought was enabling coverage on both Scala versions makes some sense. True we will only get one codecov report, but I believe we have coverage threshold check, which fails the build if the delta coverage falls below (on the other hand I haven't seen that in action for a while, so maybe we need to check the settings, not necessarily in this PR though). For those coverage checks it kind of makes sense to have them on both Scala versions.

Unfortunately our microsite is tied to only one Scala version. I propose we change it to Scala 2.13, although I am not 100% sure if sbt-microsite supports it yet.

[DieBauer]

Ok that makes sense. Not sure what the correct place would be in the travis job though

It seems that bench/test works on 2.13.0 as well. So the comment (https://github.com/typelevel/cats/blob/master/.travis.yml#L51) is not valid anymore and we can merge the jobs
with something like this?

    - &jvm_tests
      stage: test
      env: TEST="JVM tests"
      script: sbt ++$TRAVIS_SCALA_VERSION! buildJVM bench/test
      scala:
      - *scala_version_212
      - *scala_version_213

Where would coverage reporting then fit in && codecov? in this job, or still in the separate job that you mentioned?

[kailuowang]

that's good news.
yeah, let's put the coverage in the jvm_tests jobs

[kailuowang]

Looks good to me. Thanks for discovering the flag.
Just two nit comments

[kailuowang]

Thanks!

[DieBauer]

Fixed merge conflict with master.