Backport #2772 add foldRightDefer to Foldable

[gagandeepkalra]

addresses #3143

[gagandeepkalra]

java.lang.RuntimeException: Cats laws: Binary compatibility check failed!

I see other laws added without care for BC, did we add this constraint recently?

It's gonna be tricky to follow through with this.

[travisbrown]

We've always guaranteed (and checked) bincompat within 2.x for laws on mainline, and I think we should on the 2.11 branch as well (note that this kind of addition only breaks bincompat on 2.11).

My vote would be just not to include the consistency law on this branch, possibly adding a comment to the method along the lines of "Implementers are responsible for ensuring they maintain consistency with foldRight; this is not checked by laws on Scala 2.11".

[codecov-io]

Codecov Report

Merging #3338 into scala_2.11 will decrease coverage by 0.02%.
The diff coverage is 50%.

@@              Coverage Diff               @@
##           scala_2.11    #3338      +/-   ##
==============================================
- Coverage       93.46%   93.44%   -0.03%     
==============================================
  Files             385      385              
  Lines            7162     7166       +4     
  Branches          197      186      -11     
==============================================
+ Hits             6694     6696       +2     
- Misses            468      470       +2

Impacted Files	Coverage Δ
core/src/main/scala/cats/Foldable.scala	98.07% <0%> (-1.93%)	⬇️
core/src/main/scala/cats/syntax/foldable.scala	100% <100%> (ø)	⬆️

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[gagandeepkalra]

@travisbrown Think I'm a little confused, I see this added in May yet no BC problem.

yes, adding foldRightDefer would only break BC on 2.11, but how was the above law added given it is part of 2.11.

[travisbrown]

@gagandeepkalra See @kailuowang's comment on #2817:

BTW, strictly speaking this is breaking change for UnorderedFoldableLaws but it's unlikely to affect anyone and we are breaking BC in cats-laws in our next release.

And mine above:

We've always guaranteed (and checked) bincompat within 2.x for laws on mainline

That addition was first included in 2.0.0, so while it was a breaking change since 1.x on Scala 2.11, we'd decided that that was okay for cats-laws (we only guaranteed 100% bincompat across 1.x and 2.x for cats-core, etc.).

[gagandeepkalra]

I see, thank you. Apologies for not checking the PR first 😔

[travisbrown]

No worries, glad that helped!

[travisbrown]

Thanks!