add child to parent in completion context

This ensures that the positions of forced Child annotations
always happens in the source file of the parent class, and not in
another file that forced the completion to happen

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

@@ -848,7 +848,7 @@ class Namer { typer: Typer =>
       else
         try
           completeInCreationContext(denot)
-          if (denot.isCompleted) registerIfChild(denot)
+          if (denot.isCompleted) registerIfChildInCreationContext(denot)
         catch
           case ex: CompilationUnit.SuspendException =>
             val completer = SuspendCompleter()
@@ -937,10 +937,12 @@ class Namer { typer: Typer =>
         denot.markAbsent()
     end invalidateIfClashingSynthetic
 
-    /** If completed symbol is an enum value or a named class, register it as a child
+    /** Intentionally left without `using Context` parameter.
+     *  This action should be performed in the context of where the completer was created.
+     *  If completed symbol is an enum value or a named class, register it as a child
      *  in all direct parent classes which are sealed.
      */
-    def registerIfChild(denot: SymDenotation)(using Context): Unit = {
+    def registerIfChildInCreationContext(denot: SymDenotation): Unit = {
       val sym = denot.symbol
 
       def register(child: Symbol, parentCls: ClassSymbol) = {
@@ -964,7 +966,7 @@ class Namer { typer: Typer =>
       end if
     }
 
-    /** Intentionally left without `implicit ctx` parameter. We need
+    /** Intentionally left without `using Context` parameter. We need
      *  to pick up the context at the point where the completer was created.
      */
     def completeInCreationContext(denot: SymDenotation): Unit = {

tests/pos/i21154/A.scala

@@ -0,0 +1,6 @@
+import Z.*
+
+object A:
+  val a: Option[AOptions] = ???
+  val b: Option[BOptions] = ???
+  val c: Option[COptions] = ???

tests/pos/i21154/Z.scala

@@ -0,0 +1,9 @@
+//> using options -Ytest-pickler-check
+
+// in the original issue https://github.com/scala/scala3/issues/21154, the non-deterministic tasty
+// depends on the order of compilation of files, the use-site (A.scala) has to come first,
+// and the file defining the enum has to come second (Z.scala), A.scala in namer will force Z to complete.
+enum Z:
+  case AOptions()
+  case BOptions()
+  case COptions()