Fix race condition in new LazyVals

library/src/scala/runtime/LazyVals.scala

@@ -45,7 +45,8 @@ object LazyVals {
 
   /* ------------- Start of public API ------------- */
 
-  sealed trait LazyValControlState
+  // This trait extends Serializable to fix #16806 that caused a race condition
+  sealed trait LazyValControlState extends Serializable
 
   /**
    * Used to indicate the state of a lazy val that is being

project/MiMaFilters.scala

@@ -26,6 +26,11 @@ object MiMaFilters {
     ProblemFilters.exclude[MissingFieldProblem]("scala.runtime.stdLibPatches.language#experimental.into"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.stdLibPatches.language$experimental$into$"),
     // end of New experimental features in 3.3.X
+
+    // Added java.io.Serializable as LazyValControlState supertype
+    ProblemFilters.exclude[MissingTypesProblem]("scala.runtime.LazyVals$LazyValControlState"),
+    ProblemFilters.exclude[MissingTypesProblem]("scala.runtime.LazyVals$Waiting"),
+
   )
   val TastyCore: Seq[ProblemFilter] = Seq(
     ProblemFilters.exclude[DirectMissingMethodProblem]("dotty.tools.tasty.TastyBuffer.reset"),

tests/run/i16806.check

@@ -0,0 +1,2 @@
+Success
+Success

tests/run/i16806.scala

@@ -0,0 +1,42 @@
+//scalajs: --skip
+import java.util.concurrent.Semaphore
+
+object Repro {
+
+  case object DFBit
+  final class DFError extends Exception("")
+  final class DFType[+T](val value: T | DFError) extends AnyVal
+
+  def asIR(dfType: DFType[DFBit.type]): DFBit.type = dfType.value match
+  case dfTypeIR: DFBit.type => dfTypeIR
+  case err: DFError         => throw new DFError
+
+  object Holder {
+    val s = new Semaphore(1, false)
+    final lazy val Bit = {
+      s.release()
+      new DFType[DFBit.type](DFBit)
+    }
+  }
+
+  @main
+  def Test =
+    val a = new Thread() {
+      override def run(): Unit =
+        Holder.s.acquire()
+        val x = Holder.Bit.value
+        assert(x.isInstanceOf[DFBit.type])
+        println("Success")
+    }
+    val b = new Thread() {
+      override def run(): Unit =
+        Holder.s.acquire()
+        val x = Holder.Bit.value
+        assert(x.isInstanceOf[DFBit.type])
+        println("Success")
+    }
+    a.start()
+    b.start()
+    a.join(300)
+    b.join(300)
+}