UnorderedFoldable#isEmpty default implementation is incorrect

[barambani]

Fixes issue #2584 and adds some tests.

[codecov-io]

Codecov Report

Merging #2586 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2586      +/-   ##
==========================================
+ Coverage   95.14%   95.16%   +0.01%     
==========================================
  Files         361      361              
  Lines        6634     6634              
  Branches      282      294      +12     
==========================================
+ Hits         6312     6313       +1     
+ Misses        322      321       -1

Impacted Files	Coverage Δ
core/src/main/scala/cats/UnorderedFoldable.scala	92.85% <100%> (+7.14%)	⬆️

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[kailuowang]

Nice! Thanks for the update of the tests!

[barambani]

👍

[LukaJCB]

Tiny tiny nitpick but nonEmpty now negates exists twice. Maybe we should define nonEmpty as exists(fa)(Function.const(true)) and then isEmpty as !nonEmpty WDYT? :)

[barambani]

Actually I started from that :), but then I changed it as I had the impression it was reading better in my head (not exists instead of not non empty). But I don't really love my version either, as you said double negates, and even if it's simple logic, that makes reasoning convoluted and unnatural. I'm happy to change it 👍.

[LukaJCB]

Thanks a ton for fixing this so quickly! :)

[barambani]

thank you for the great help. As usual 👍