fix: direct recursion

fixes that recursive structures could lead to an infinite loop at
runtime if the very first field of a product would recur to the
"parent" sum (because of a lack of lazyness).

src/main/derived/ArbitraryDeriving.scala

@@ -21,20 +21,24 @@ private object Gens:
 
   def apply[T](gen: Gen[T]): Gens[T] = Gens(List(gen))
 
-  inline private def tupleInstance[T <: Tuple]: Gens[T] =
+  inline private def tupleInstance[T <: Tuple](isHead: Boolean): Gens[T] =
     inline erasedValue[T] match
       case _: EmptyTuple =>
         Gens(Gen.const(EmptyTuple.asInstanceOf[T]))
       case _: (t *: ts) =>
         val gen: Gen[T] =
           for {
-            tVal <- scalacheck.anyGivenArbitrary[t].arbitrary
-            tsVal <- tupleInstance[ts].gen
+            tVal <-
+              // only for the very first member of a product, some extra lazyness is needed to
+              // ensure we don't end up in an endless loop in case of recursive structures
+              if (isHead) Gen.lzy(scalacheck.anyGivenArbitrary[t].arbitrary)
+              else scalacheck.anyGivenArbitrary[t].arbitrary
+            tsVal <- tupleInstance[ts](false).gen
           } yield (tVal *: tsVal).asInstanceOf[T]
         Gens(gen)
 
   inline def productInstance[T](p: Mirror.ProductOf[T]): Gens[T] =
-    Gens(tupleInstance[p.MirroredElemTypes].gen.map(p.fromProduct(_)))
+    Gens(tupleInstance[p.MirroredElemTypes](true).gen.map(p.fromProduct(_)))
 
   private inline def summonSumInstances[T, Elems <: Tuple]: List[Gens[T]] =
     inline erasedValue[Elems] match

src/test/ArbitraryDerivingSuite.scala

@@ -82,6 +82,10 @@ class ArbitraryDerivingSuite extends munit.FunSuite:
     equalValues(MaybeMaybeList.expectedGen[Int])
   }
 
+  test("supports direct recursion)") {
+    equalValues(DirectRecursion.expectedGen)
+  }
+
   // not a hard requirement (just guarding against accidental worsening by refactoring)
   test("supports case classes with up to 26 fields (if -Xmax-inlines=32)") {
     summon[Arbitrary[MaxCaseClass]]

src/test/CogenDerivingSuite.scala

@@ -73,6 +73,10 @@ class CogenDerivingSuite extends munit.ScalaCheckSuite:
     equalValues(MaybeMaybeList.expectedCogen[Int])
   }
 
+  test("given derivation supports direct recursion") {
+    equalValues(DirectRecursion.expectedCogen)
+  }
+
   test("enables derivation of Arbitrary instances for functions") {
     val arbFunction1: Arbitrary[ComplexADTWithNestedMembers => ABC] =
       summon

src/test/ShrinkDerivingSuite.scala

@@ -71,6 +71,10 @@ class ShrinkDerivingSuite extends munit.ScalaCheckSuite:
     equalValues(MaybeMaybeList.expectedShrink[Int])
   }
 
+  property("supports direct recursion") {
+    equalValues(DirectRecursion.expectedShrink)
+  }
+
   // seems there is no feasible way to get up to par with ArbitraryDeriving, so this is just a
   // guard against making things even worse
   test("supports case classes with up to 25 fields (if -Xmax-inlines=32)") {

src/test/test_classes.scala

@@ -471,6 +471,46 @@ object MaybeMaybeList:
       mml => (mml.head, mml.tail)
     )(shrinkTuple)
 
+enum DirectRecursion:
+  case Continue(next: DirectRecursion)
+  case Stop
+
+object DirectRecursion:
+
+  def expectedGen: Gen[DirectRecursion] =
+    Gen.oneOf(Gen.lzy(expectedGen.map(Continue(_))), Gen.const(Stop))
+
+  def expectedCogen: Cogen[DirectRecursion] =
+    Cogen { (seed, value) =>
+      value match
+        case Continue(dr) =>
+          perturb(
+            perturb[Unit](
+              perturb[DirectRecursion](
+                seed,
+                dr
+              )(expectedCogen),
+              ()
+            ),
+            0
+          )
+        case Stop =>
+          perturbSingletonInSum(1, seed, Stop)
+    }
+
+  @annotation.nowarn("msg=Stream .* is deprecated")
+  def expectedShrink: Shrink[DirectRecursion] =
+    Shrink {
+      case c: Continue =>
+        Shrink
+          .xmap[DirectRecursion, Continue](
+            Continue.apply,
+            _.next
+          )(expectedShrink)
+          .shrink(c)
+      case Stop => Stream.empty
+    }
+
 // format: off
 case class MaxCaseClass(
   a1: Int, b1: Int, c1: Int, d1: Int, e1: Int, f1: Int, g1: Int, h1: Int, i1: Int, j1: Int,