About ambiguous implicit instances in Cats module

[Iltotore]

Summoning again @gvolpe because I forgot to ask it when the PR got merged: why do we need to make our given Hash low priority?

Is this because it can conflict with Eq since Hash <: Eq? If yes, then shouldn't the same problem occur with Ord and Eq, Group and Monoid etc... ?

[gvolpe]

The first reason is the clash with the Eq instance, that's correct. But I think it has to do with how it's defined:

trait Hash[@sp A] extends Any with Eq[A] with Serializable

Versus

trait Order[@sp A] extends Any with PartialOrder[A]

trait PartialOrder[@sp A] extends Any with Eq[A] 

trait Eq[@sp A] extends Any with Serializable

Hash extends Serializable directly, and that seems to affect implicit resolution, but that's just a wild guess. You can see how Eq also extends Serializable, but in the eyes of both Order and PartialOrder it's another layer down in the mixin.

I only know that the low-priority pattern is quite common for typeclass instances, so I just moved this one there and everything else compiled.

[Iltotore]

but in the eyes of both Order and PartialOrder it's another layer down in the mixin.

So there is no conflict between Eq and Order/PartialOrder? That's my actual question to know if these instances should also be moved or not.

[gvolpe]

There are no conflicts at all. It's all checked in the tests at compile time.