Merge pull request scala#6004 from retronym/ticket/10423

Don't assume final member class in another comp. unit will stay final

src/compiler/scala/tools/nsc/transform/Constructors.scala

@@ -764,7 +764,7 @@ abstract class Constructors extends Statics with Transform with TypingTransforme
           primaryConstrBody.expr)
       })
 
-      if (omittableAccessor.exists(_.isOuterField) && !constructorStats.exists(_.exists { case i: Ident if i.symbol.isOuterParam => true; case _ => false}))
+      if ((exitingPickler(clazz.isAnonymousClass) || clazz.originalOwner.isTerm) && omittableAccessor.exists(_.isOuterField) && !constructorStats.exists(_.exists { case i: Ident if i.symbol.isOuterParam => true; case _ => false}))
         primaryConstructor.symbol.updateAttachment(OuterArgCanBeElided)
 
       val constructors = primaryConstructor :: auxConstructors

test/files/run/t10423/A_1.scala

@@ -0,0 +1,11 @@
+class Outer {
+  final class Inner {
+    def foo: Unit = ()
+  }
+}
+object Test {
+  def main(args: Array[String]): Unit = {
+    val o = new Outer
+    new o.Inner().foo
+  }
+}

test/files/run/t10423/A_2.scala

@@ -0,0 +1,6 @@
+class Outer {
+  class Inner {
+    def foo: Unit = assert(Outer.this ne null)
+  }
+}
+