prevent derivation of invalid seed via Seed.apply(0, 0, 0, 0)

typelevel/scalacheck#674
spotify · Aug 13, 2020 · fe0b747 · fe0b747
1 parent 67d9054
commit fe0b747
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Showing 3 changed files with 24 additions and 14 deletions.
diff --git a/scalacheck/src/main/scala/magnolify/scalacheck/semiauto/ArbitraryDerivation.scala b/scalacheck/src/main/scala/magnolify/scalacheck/semiauto/ArbitraryDerivation.scala
@@ -18,6 +18,7 @@ package magnolify.scalacheck.semiauto
 
 import magnolia._
 import magnolify.shims.Monadic
+import org.scalacheck.rng.Seed
 import org.scalacheck.{Arbitrary, Gen}
 
 import scala.language.experimental.macros
@@ -36,14 +37,20 @@ object ArbitraryDerivation {
   }
 
   def dispatch[T: Fallback](sealedTrait: SealedTrait[Typeclass, T]): Typeclass[T] = Arbitrary {
-    Gen.sized { size =>
-      if (size > 0) {
-        Gen.resize(
-          size - 1,
-          Gen.oneOf(sealedTrait.subtypes.map(_.typeclass.arbitrary)).flatMap(identity)
-        )
-      } else {
-        implicitly[Fallback[T]].get
+    if (sealedTrait.typeName.full == classOf[Seed].getCanonicalName) {
+      // Prevent derivation of invalid seed via `Seed.apply(0, 0, 0, 0)`
+      // https://github.com/typelevel/scalacheck/pull/674
+      Arbitrary.arbLong.arbitrary.map(Seed(_)).asInstanceOf[Gen[T]]
+    } else {
+      Gen.sized { size =>
+        if (size > 0) {
+          Gen.resize(
+            size - 1,
+            Gen.oneOf(sealedTrait.subtypes.map(_.typeclass.arbitrary)).flatMap(identity)
+          )
+        } else {
+          implicitly[Fallback[T]].get
+        }
       }
     }
   }

diff --git a/scalacheck/src/test/scala/magnolify/scalacheck/test/ArbitraryDerivationSuite.scala b/scalacheck/src/test/scala/magnolify/scalacheck/test/ArbitraryDerivationSuite.scala
@@ -35,8 +35,7 @@ class ArbitraryDerivationSuite extends MagnolifySuite {
     val prms = Gen.Parameters.default
     // `forAll(Gen.listOfN(10, g))` fails for `Repeated` & `Collections` when size parameter <= 1
     property(s"$name.uniqueness") {
-      Prop.forAll { l: Long =>
-        val seed = Seed(l) // prevent Magnolia from deriving `Seed`
+      Prop.forAll { seed: Seed =>
         val xs = Gen.listOfN(10, g)(prms, seed).get
         xs.iterator.map(f).toSet.size > 1
       }
@@ -91,4 +90,10 @@ class ArbitraryDerivationSuite extends MagnolifySuite {
 
   test[Shape]
   test[Color]
+
+  property("Seed") {
+    Prop.forAll { seed: Seed =>
+      seed.next != seed
+    }
+  }
 }
diff --git a/scalacheck/src/test/scala/magnolify/scalacheck/test/CogenDerivationSuite.scala b/scalacheck/src/test/scala/magnolify/scalacheck/test/CogenDerivationSuite.scala
@@ -34,14 +34,12 @@ class CogenDerivationSuite extends MagnolifySuite {
     val name = className[T]
     implicit val arbList: Arbitrary[List[T]] = Arbitrary(Gen.listOfN(10, arb.arbitrary))
     property(s"$name.uniqueness") {
-      Prop.forAll { (l: Long, xs: List[T]) =>
-        val seed = Seed(l) // prevent Magnolia from deriving `Seed`
+      Prop.forAll { (seed: Seed, xs: List[T]) =>
         xs.map(co.perturb(seed, _)).toSet.size == xs.map(f).toSet.size
       }
     }
     property(s"$name.consistency") {
-      Prop.forAll { (l: Long, x: T) =>
-        val seed = Seed(l) // prevent Magnolia from deriving `Seed`
+      Prop.forAll { (seed: Seed, x: T) =>
         co.perturb(seed, x) == co.perturb(seed, x)
       }
     }