Add ReferentialEq[A] to Alley Cats (#3925)

* Add Eq.fromReferentialEquals

* Revert "Add Eq.fromReferentialEquals"

This reverts commit 63f7272.

* Add ReferentialEq to Alley Cats

* Add tests for ReferentialEq

* Improved arbitrary

Author: armanbilge

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

@@ -0,0 +1,13 @@
+package alleycats
+
+import cats.Eq
+
+/**
+ * An `Eq[A]` that delegates to referential equality (`eq`).
+ * Note that it is not referentially transparent!
+ */
+object ReferentialEq {
+  def apply[A <: AnyRef]: Eq[A] = new Eq[A] {
+    def eqv(x: A, y: A) = x eq y
+  }
+}

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

@@ -0,0 +1,44 @@
+package alleycats.tests
+
+import alleycats.ReferentialEq
+import cats.kernel.Eq
+import cats.kernel.laws.discipline._
+import cats.kernel.laws.EqLaws
+import org.scalacheck.Arbitrary
+import org.scalacheck.Gen
+import org.scalacheck.Prop.forAll
+import org.typelevel.discipline.Laws
+
+class ReferentialEqSuite extends AlleycatsSuite {
+
+  class ReferentialEqTests[A](eq: Eq[A]) extends Laws {
+    def laws = EqLaws(eq)
+
+    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 _)
+      )
+    }
+  }
+
+  implicit val arbObject: Arbitrary[Object] =
+    // with some probability we select from a small set of objects
+    // otherwise make a totally new one
+    // courtesy of @johnynek
+    Arbitrary(
+      Gen.oneOf(
+        Gen.oneOf(List.fill(5)(new Object)),
+        Arbitrary.arbUnit.arbitrary.map(_ => new Object)
+      )
+    )
+
+  implicit val eqObject: Eq[Object] = ReferentialEq[Object]
+
+  checkAll("ReferentialEq[Object]", new ReferentialEqTests(ReferentialEq[Object]).eqv)
+
+}