Make myNotNullInfos field private

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

@@ -785,7 +785,7 @@ class Namer { typer: Typer =>
 
     protected def localContext(owner: Symbol): FreshContext = ctx.fresh.setOwner(owner).setTree(original)
 
-    var myNotNullInfos: List[NotNullInfo] | Null = null
+    private var myNotNullInfos: List[NotNullInfo] | Null = null
 
     /** The context with which this completer was created */
     given creationContext[T]: Context =
@@ -794,6 +794,9 @@ class Namer { typer: Typer =>
     // make sure testing contexts are not captured by completers
     assert(!ictx.reporter.isInstanceOf[ExploringReporter])
 
+    def setNotNullInfos(infos: List[NotNullInfo]): Unit =
+      myNotNullInfos = infos
+
     protected def typeSig(sym: Symbol): Type = original match
       case original: ValDef =>
         if (sym.is(Module)) moduleValSig(sym)

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

@@ -3361,11 +3361,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     mdef.getAttachment(SymOfTree) match {
       case Some(sym) => sym.infoOrCompleter match {
         case completer: Namer#Completer =>
-          if (completer.creationContext.notNullInfos ne ctx.notNullInfos)
+          if completer.creationContext.notNullInfos ne ctx.notNullInfos then
             // The RHS of a val def should know about not null facts established
             // in preceding statements (unless the DefTree is completed ahead of time,
             // then it is impossible).
-            completer.myNotNullInfos = ctx.notNullInfos
+            completer.setNotNullInfos(ctx.notNullInfos)
           true
         case _ =>
           // If it has been completed, then it must be because there is a forward reference