Fix #9115: Make Java-defined fields effectively final

Author: odersky

compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -308,7 +308,7 @@ object Flags {
   val (Given @ _, _, _) = newFlags(29, "given")
 
   /** Symbol is defined by a Java class */
-  val (JavaDefined @ _, _, _) = newFlags(30, "<java>")
+  val (JavaDefined @ _, JavaDefinedVal @ _, _) = newFlags(30, "<java>")
 
   /** Symbol is implemented as a Java static */
   val (JavaStatic @ _, JavaStaticTerm @ _, JavaStaticType @ _) = newFlags(31, "<static>")

compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -1158,6 +1158,7 @@ object SymDenotations {
     final def isEffectivelyFinal(implicit ctx: Context): Boolean =
       isOneOf(EffectivelyFinalFlags)
       || is(Inline, butNot = Deferred)
+      || is(JavaDefinedVal, butNot = Method)
       || !owner.isExtensibleClass
 
     /** A class is effectively sealed if has the `final` or `sealed` modifier, or it

tests/neg/i9115/A.java

@@ -0,0 +1,3 @@
+public class A {
+  public String foo = "A";
+}

tests/neg/i9115/B.scala

@@ -0,0 +1,3 @@
+class B extends A {
+  override val foo: String = "B" // error
+}