Fix "forward reference from self constructor" checking

The previous check did not consider the case where the self constructor
was itself a block that originated from named and default parameters in
the actual self constructor call. That gave a false negative for some code
in akka. The negative was not discovered before since we did not propagate
the correct context information into the expression of a block.

compiler/src/dotty/tools/dotc/transform/ForwardDepChecks.scala

@@ -89,19 +89,38 @@ class ForwardDepChecks extends MiniPhase:
     tree
   }
 
-  override def transformApply(tree: Apply)(using Context): Apply = {
-    if (isSelfConstrCall(tree)) {
-      assert(currentLevel.isInstanceOf[LevelInfo], s"${ctx.owner}/" + i"$tree")
-      val level = currentLevel.asInstanceOf[LevelInfo]
-      if (level.maxIndex > 0) {
-        // An implementation restriction to avoid VerifyErrors and lazyvals mishaps; see SI-4717
-        report.debuglog("refsym = " + level.refSym)
-        report.error("forward reference not allowed from self constructor invocation",
-          ctx.source.atSpan(level.refSpan))
-      }
-    }
+  /** Check that self constructor call does not contain references to vals or defs
+   *  defined later in the secondary constructor's right hand side. This is tricky
+   *  since the complete self constructor might itself be a block that originated from
+   *  expanding named and default parameters. In that case we have to go outwards
+   *  and find the enclosing expression that consists of that block. Test cases in
+   *  {pos,neg}/complex-self-call.scala.
+   */
+  private def checkSelfConstructorCall()(using Context): Unit =
+    // Find level info corresponding to constructor's RHS. This is the info of the
+    // outermost context that has the constructor as owner and that has a LevelInfo.
+    def rhsLevelInfo(using ctx: Context): OptLevelInfo =
+      if ctx.outer.owner == ctx.owner then
+        rhsLevelInfo(using ctx.outer) match
+          case level: LevelInfo => level
+          case _ => currentLevel
+      else currentLevel
+
+    rhsLevelInfo match
+      case level: LevelInfo =>
+        if level.maxIndex > 0 then
+          report.debuglog("refsym = " + level.refSym.showLocated)
+          report.error("forward reference not allowed from self constructor invocation",
+            ctx.source.atSpan(level.refSpan))
+      case _ =>
+        assert(false, s"${ctx.owner.showLocated}")
+  end checkSelfConstructorCall
+
+  override def transformApply(tree: Apply)(using Context): Apply =
+    if (isSelfConstrCall(tree))
+      assert(ctx.owner.isConstructor)
+      checkSelfConstructorCall()
     tree
-  }
 
   override def transformNew(tree: New)(using Context): New = {
     currentLevel.enterReference(tree.tpe.typeSymbol, tree.span)

tests/neg/complex-self-call.scala

@@ -0,0 +1,30 @@
+// An example extracted from akka that demonstrated a spurious
+// "forward reference not allowed from self constructor invocation" error.
+class Resizer
+class SupervisorStrategy
+class Pool
+object Pool:
+  def defaultSupervisorStrategy: SupervisorStrategy = ???
+object Dispatchers:
+  def DefaultDispatcherId = ???
+object Resizer:
+  def fromConfig(config: Config): Option[Resizer] = ???
+
+class Config:
+  def getInt(str: String): Int = ???
+  def hasPath(str: String): Boolean = ???
+
+final case class BroadcastPool(
+    nrOfInstances: Int,
+    val resizer: Option[Resizer] = None,
+    val supervisorStrategy: SupervisorStrategy = Pool.defaultSupervisorStrategy,
+    val routerDispatcher: String = Dispatchers.DefaultDispatcherId,
+    val usePoolDispatcher: Boolean = false)
+    extends Pool:
+
+  def this(config: Config) =
+    this(
+      nrOfInstances = config.getInt("nr-of-instances"),
+      resizer = resiz,  // error: forward reference not allowed from self constructor invocation
+      usePoolDispatcher = config.hasPath("pool-dispatcher"))
+    def resiz = Resizer.fromConfig(config)

tests/pos/complex-self-call.scala

@@ -0,0 +1,29 @@
+// An example extracted from akka that demonstrated a spurious
+// "forward reference not allowed from self constructor invocation" error.
+class Resizer
+class SupervisorStrategy
+class Pool
+object Pool:
+  def defaultSupervisorStrategy: SupervisorStrategy = ???
+object Dispatchers:
+  def DefaultDispatcherId = ???
+object Resizer:
+  def fromConfig(config: Config): Option[Resizer] = ???
+
+class Config:
+  def getInt(str: String): Int = ???
+  def hasPath(str: String): Boolean = ???
+
+final case class BroadcastPool(
+    nrOfInstances: Int,
+    val resizer: Option[Resizer] = None,
+    val supervisorStrategy: SupervisorStrategy = Pool.defaultSupervisorStrategy,
+    val routerDispatcher: String = Dispatchers.DefaultDispatcherId,
+    val usePoolDispatcher: Boolean = false)
+    extends Pool:
+
+  def this(config: Config) =
+    this(
+      nrOfInstances = config.getInt("nr-of-instances"),
+      resizer = Resizer.fromConfig(config),
+      usePoolDispatcher = config.hasPath("pool-dispatcher"))