SetPartialOrder[A] uses the universal == instead of Eq

[ctongfei]

https://github.com/typelevel/cats/blob/master/kernel/src/main/scala/cats/kernel/instances/set.scala#L22

This instance should require an implicit Eq[A] and use this instance for containment partial order / set equivalence.

[kailuowang]

Would it have surprising effects when Set itself doesn't use Eq[A]?

[ctongfei]

@kailuowang OK I get it. For the instances for scala.collection.{Set/Map}, use the standard universal equals on keys. Thanks!

[johnynek]

yeah, I think if you use Set and Map they have to use universal equality (on keys for map at least), so doing this differently would really be a bug.

[ctongfei]

@johnynek You are right. The equality on them must be the universal one. Thanks!