diff --git a/build.sbt b/build.sbt
index a2bba02..01a8fc3 100644
--- a/build.sbt
+++ b/build.sbt
@@ -1,6 +1,6 @@
 import com.typesafe.tools.mima.core.*
 
-val scala3Version = "3.3.8"
+val scala3Version = "3.9.0-RC3"
 
 ThisBuild / organization := "org.typelevel"
 ThisBuild / tlBaseVersion := "3.6"
@@ -110,7 +110,9 @@ lazy val typeable = crossProject(JSPlatform, JVMPlatform, NativePlatform)
     mimaBinaryIssueFilters ++= Seq(
       // Ops was replaced by extension methods in https://github.com/typelevel/shapeless-3/pull/1
       ProblemFilters.exclude[DirectMissingMethodProblem]("shapeless3.typeable.syntax#typeable.Ops"),
-      ProblemFilters.exclude[MissingClassProblem]("shapeless3.typeable.syntax$typeable$Ops")
+      ProblemFilters.exclude[MissingClassProblem]("shapeless3.typeable.syntax$typeable$Ops"),
+      // Changed on upgrade to Scala 3.9
+      ProblemFilters.exclude[DirectMissingMethodProblem]("shapeless3.typeable.Typeable.<clinit>")
     )
   )
 
diff --git a/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala b/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala
index 455ae0c..b1e8701 100644
--- a/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala
+++ b/modules/deriving/src/test/scala/shapeless3/deriving/deriving.scala
@@ -424,7 +424,7 @@ class DerivationTests:
     assert(v5.show == "Order(item: String, quantity: Int)")
 
     val v6 = ShowType[Order[Option]]
-    assert(v6.show == "Order(item: None | Some(value: String), quantity: None | Some(value: Int))")
+    assert(v6.show == "Order(item: Some(value: String) | None, quantity: Some(value: Int) | None)")
 
   @Test
   def read(): Unit =
diff --git a/modules/deriving/src/test/scala/shapeless3/deriving/kinds.scala b/modules/deriving/src/test/scala/shapeless3/deriving/kinds.scala
index 5192b72..56d86bd 100644
--- a/modules/deriving/src/test/scala/shapeless3/deriving/kinds.scala
+++ b/modules/deriving/src/test/scala/shapeless3/deriving/kinds.scala
@@ -81,11 +81,10 @@ object K21
     ): R =
       inst.erasedFold2f(x, y)(g.asInstanceOf)(f.asInstanceOf).asInstanceOf
 
-type IdRef[T <: AnyRef] = T
 type HeadRef[T] <: AnyRef = T match
-  case IdRef[h] *: _ => h
+  case AnyRef *: _ => Tuple.Head[T] & AnyRef
 
-object K1Ref extends Kind[AnyRef, Tuple, IdRef, HeadRef, Kinds.Tail]:
+object K1Ref extends Kind[AnyRef, Tuple, Id, HeadRef, Kinds.Tail]:
   extension [F[_ <: AnyRef], T <: AnyRef](inst: ProductInstances[F, T])
     inline def foldLeft2[Acc](x: T, y: T)(i: Acc)(f: [t <: AnyRef] => (Acc, F[t], t, t) => CompleteOr[Acc]): Acc =
       inst.erasedFoldLeft2(x, y)(i)(f.asInstanceOf).asInstanceOf
diff --git a/modules/typeable/src/test/scala/shapeless3/typeable/typeable.scala b/modules/typeable/src/test/scala/shapeless3/typeable/typeable.scala
index 8ebebcb..f589cca 100644
--- a/modules/typeable/src/test/scala/shapeless3/typeable/typeable.scala
+++ b/modules/typeable/src/test/scala/shapeless3/typeable/typeable.scala
@@ -16,6 +16,8 @@
 
 package shapeless3.typeable
 
+import scala.compiletime.uninitialized
+
 class TypeableTests:
   import TypeableTests.*
   import org.junit.Assert.*
@@ -389,7 +391,7 @@ class TypeableTests:
   trait Tc[A]
   case class Gen2[A: Tc](i: Int)
   case class Gen3[A](i: Int):
-    var a: A = _
+    var a: A = uninitialized
   abstract class Abs[A](a: A):
     val x: A = a
   case class Gen4[A](i: Int)(a: A) extends Abs[A](a)
@@ -493,7 +495,7 @@ class TypeableTests:
     val bi: Bar[Int] = Bar(23)
     assertEquals("Typeable[Bar[Int]]", typeableString(bi))
 
-    val i1: A with B = new C
+    val i1: A & B = new C
     assertEquals("Typeable[A & B]", typeableString(i1))
     assertEquals("Typeable[A]", typeableString(new A {}))
     assertEquals("Typeable[A]", Typeable[A].toString)
@@ -593,7 +595,7 @@ class TypeableTests:
     object X:
       case class A()
       case class B(a: A):
-        private[this] val aa = a
+        private val aa = a
     object Test:
       Typeable[X.B]