addressing PR comments

Author: SimY4

alleycats-core/src/main/scala/alleycats/ReferentialEq.scala

@@ -6,12 +6,12 @@ import cats.Eq
  * An `Eq[A]` that delegates to referential equality (`eq`).
  * Note that it is not referentially transparent!
  */
-class ReferentialEq[A <: AnyRef] extends Eq[A] {
-  def eqv(x: A, y: A) = x eq y
+trait ReferentialEq[A <: AnyRef] extends Eq[A] {
+  def eqv(x: A, y: A): Boolean = x eq y
 }
 
 object ReferentialEq {
-  private[this] val referentialEq: Eq[AnyRef] = new ReferentialEq[AnyRef]
+  private[this] val referentialEq: Eq[AnyRef] = new ReferentialEq[AnyRef] {}
 
   def apply[A <: AnyRef]: Eq[A] = referentialEq.asInstanceOf[Eq[A]]
 }

alleycats-core/.jvm/src/main/scala/alleycats/SystemIdentityHash.scala renamed to alleycats-core/src/main/scala/alleycats/SystemIdentityHash.scala

@@ -2,12 +2,12 @@ package alleycats
 
 import cats.Hash
 
-class SystemIdentityHash[A <: AnyRef] extends ReferentialEq[A] with Hash[A] {
+trait SystemIdentityHash[A <: AnyRef] extends ReferentialEq[A] with Hash[A] {
   override def hash(a: A): Int = java.lang.System.identityHashCode(a)
 }
 
 object SystemIdentityHash {
-  private[this] val identityHash: Hash[AnyRef] = new SystemIdentityHash[AnyRef]
+  private[this] val identityHash: Hash[AnyRef] = new SystemIdentityHash[AnyRef] {}
 
   def apply[A <: AnyRef]: Hash[A] = identityHash.asInstanceOf[Hash[A]]
-}
+}

alleycats-laws/src/main/scala/alleycats/laws/discipline/ReferentialEqTests.scala

@@ -0,0 +1,29 @@
+package alleycats.laws.discipline
+
+import cats.kernel.Eq
+import cats.kernel.laws.EqLaws
+import cats.kernel.laws.discipline._
+import org.scalacheck.Arbitrary
+import org.scalacheck.Prop.forAll
+import org.typelevel.discipline.Laws
+
+trait ReferentialEqTests[A] extends Laws {
+  def laws: EqLaws[A]
+
+  def eqv(implicit arbA: Arbitrary[A]): RuleSet = {
+    implicit val eqA: Eq[A] = laws.E
+    new DefaultRuleSet(
+      "referentialEq",
+      None,
+      "reflexivity eq" -> forAll(laws.reflexivityEq _),
+      "symmetry eq" -> forAll(laws.symmetryEq _),
+      "transitivity eq" -> forAll(laws.transitivityEq _)
+    )
+  }
+}
+
+object ReferentialEqTests {
+  def apply[A: Eq]: ReferentialEqTests[A] = new ReferentialEqTests[A] {
+    override def laws: EqLaws[A] = EqLaws[A]
+  }
+}

alleycats-laws/src/main/scala/alleycats/laws/discipline/SystemIdentityHashTests.scala

@@ -0,0 +1,28 @@
+package alleycats.laws.discipline
+
+import cats.kernel.{Eq, Hash}
+import cats.kernel.laws.HashLaws
+import cats.kernel.laws.discipline._
+import org.scalacheck.Arbitrary
+import org.scalacheck.Prop.forAll
+
+import scala.util.hashing.Hashing
+
+trait SystemIdentityHashTests[A] extends ReferentialEqTests[A] {
+  def laws: HashLaws[A]
+
+  def hash(implicit arbA: Arbitrary[A], eqA: Eq[A], hashA: Hashing[A]): RuleSet =
+    new DefaultRuleSet(
+      "systemIdentityHash",
+      Some(eqv),
+      "hash compatibility" -> forAll(laws.hashCompatibility _),
+      "same as universal hash" -> forAll(laws.sameAsUniversalHash _),
+      "same as scala hashing" -> forAll((x: A, y: A) => laws.sameAsScalaHashing(x, y, hashA))
+    )
+}
+
+object SystemIdentityHashTests {
+  def apply[A: Hash]: SystemIdentityHashTests[A] = new SystemIdentityHashTests[A] {
+    override def laws: HashLaws[A] = HashLaws[A]
+  }
+}

alleycats-tests/shared/src/test/scala/alleycats/tests/AlleycatsSuite.scala

@@ -4,7 +4,6 @@ import alleycats.std.MapInstances
 import cats._
 import cats.instances.all._
 import org.scalacheck.{Arbitrary, Gen}
-import org.scalacheck.Arbitrary.arbitrary
 import org.scalacheck.Test.Parameters
 
 /**
@@ -29,11 +28,4 @@ sealed trait TestInstances {
         Arbitrary.arbUnit.arbitrary.map(_ => new Object)
       )
     )
-
-  implicit val arbObjectF: Arbitrary[Object => Object] =
-    Arbitrary {
-      for {
-        obj <- arbitrary[Object]
-      } yield _ => obj
-    }
 }

alleycats-tests/shared/src/test/scala/alleycats/tests/ReferentialEqSuite.scala

@@ -1,11 +1,11 @@
 package alleycats.tests
 
 import alleycats.ReferentialEq
+import alleycats.laws.discipline.ReferentialEqTests
 import cats.kernel.Eq
-import cats.kernel.laws.discipline._
 
 class ReferentialEqSuite extends AlleycatsSuite {
   implicit val eqObject: Eq[Object] = ReferentialEq[Object]
 
-  checkAll("ReferentialEq[Object]", EqTests[Object].eqv)
+  checkAll("ReferentialEq[Object]", ReferentialEqTests[Object].eqv)
 }

alleycats-tests/jvm/src/test/scala/alleycats/tests/SystemIdentityHashSuite.scala renamed to alleycats-tests/shared/src/test/scala/alleycats/tests/SystemIdentityHashSuite.scala

@@ -1,11 +1,11 @@
 package alleycats.tests
 
 import alleycats.SystemIdentityHash
+import alleycats.laws.discipline.SystemIdentityHashTests
 import cats.kernel.Hash
-import cats.kernel.laws.discipline._
 
 class SystemIdentityHashSuite extends AlleycatsSuite {
   implicit val hashObject: Hash[Object] = SystemIdentityHash[Object]
 
-  checkAll("SystemIdentityHash[Object]", HashTests[Object].hash)
-}
+  checkAll("SystemIdentityHash[Object]", SystemIdentityHashTests[Object].hash)
+}