Fixes #24414: Fix StaleSymbolException in Namer.addChild after compilation suspension

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -536,17 +536,34 @@ class Namer { typer: Typer =>
     // to just prepending the new Child annotation.
     def isReady(ann: Annotation): Boolean =
       ann.symbol == defn.ChildAnnot && !ann.isEvaluating
+    // Forcing a Child annotation may throw `StaleSymbol` if it refers to a
+    // symbol from a previous run that is no longer valid. This happens when
+    // compilation was suspended and the child class is re-typechecked in the
+    // current run, leaving an outdated Child annotation on the parent (see
+    // tests/pos/i24414). Treat such annotations as stale and drop them.
+    def childOf(ann: Annotation): Option[Symbol] =
+      try Annotation.Child.unapply(ann)
+      catch case _: StaleSymbol => None
     def insertInto(annots: List[Annotation]): List[Annotation] =
       annots.find(isReady) match {
-        case Some(Annotation.Child(other)) if other.span.exists && childStart <= other.span.start =>
-          if (child == other)
-            annots // can happen if a class has several inaccessible children
-          else {
-            assert(childStart != other.span.start || child.source != other.source, i"duplicate child annotation $child / $other")
-            val (prefix, otherAnnot :: rest) = annots.span(ann => !isReady(ann)): @unchecked
-            prefix ::: otherAnnot :: insertInto(rest)
+        case Some(ann) =>
+          childOf(ann) match {
+            case Some(other) if other.span.exists && childStart <= other.span.start =>
+              if (child == other)
+                annots // can happen if a class has several inaccessible children
+              else {
+                assert(childStart != other.span.start || child.source != other.source, i"duplicate child annotation $child / $other")
+                val (prefix, otherAnnot :: rest) = annots.span(a => (a ne ann)): @unchecked
+                prefix ::: otherAnnot :: insertInto(rest)
+              }
+            case None =>
+              // Drop the stale Child annotation and continue searching.
+              val (prefix, rest) = annots.span(a => (a ne ann))
+              prefix ::: insertInto(rest.tail)
+            case _ =>
+              Annotation.Child(child, cls.span.startPos) :: annots
           }
-        case _ =>
+        case None =>
           Annotation.Child(child, cls.span.startPos) :: annots
       }
     cls.annotations = insertInto(cls.annotations)

tests/pos/i24414/CloudEventKey_2.scala

@@ -0,0 +1 @@
+object CloudEventKey extends RefinedType[String, MessageKey]

tests/pos/i24414/Lib_1.scala

@@ -0,0 +1,12 @@
+// https://github.com/scala/scala3/issues/24414
+trait Constraint[A, C]:
+  inline def test: Boolean
+
+final class RuntimeConstraint[A, C](_test: Boolean)
+object RuntimeConstraint:
+  inline given fromConstraint[A, C](using c: Constraint[A, C]): RuntimeConstraint[A, C] =
+    new RuntimeConstraint[A, C](c.test)
+
+sealed trait Refined[A, C](using _rtc: RuntimeConstraint[A, C])
+
+trait RefinedType[A, C] extends Refined[A, C]

tests/pos/i24414/MessageKey_2.scala

@@ -0,0 +1,9 @@
+import scala.quoted.*
+
+final class MessageKey
+
+object MessageKey:
+  inline given Constraint[String, MessageKey] with
+    override inline def test: Boolean = ${ check() }
+
+  def check()(using Quotes): Expr[Boolean] = Expr(true)