Unlawful Hash instances for Function0

[travisbrown]

I just noticed that Hash[() => A] doesn't match hashCode, and (possibly worse) it evaluates the function:

scala> val thunk = () => "abcd"
thunk: () => String = $$Lambda$4719/1999191506@1ef9f59

scala> thunk.hashCode
res0: Int = 32481113

scala> import cats.implicits._
import cats.implicits._

scala> thunk.hash
res1: Int = 2987074

scala> "abcd".hashCode
res2: Int = 2987074

We should decide whether / how to handle this and add something like this to LawTests in the kernel-laws tests:

checkAll("Hash[() => Int]", HashTests[() => Int].hash)

(I confirmed that it fails now.)

[kailuowang]

Any reason why this is closed?

[travisbrown]

@kailuowang I'm not sure it's unintended / not the right behavior. Will take a closer look tomorrow.

[kailuowang]

thanks! FWIW, this is the PR that added the code. #1712