Backport "add child to parent in completion context" to 3.5.2 (#21460)

Backports #21214 to the 3.5.2 branch.

PR submitted by the release tooling.
[skip ci]

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()