Closes #4431. Review by extempore.

3 files changed, 30 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index 18413de51f..3eb5107870 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -550,6 +550,13 @@ abstract class RefChecks extends InfoTransform {
 
         if (abstractErrors.nonEmpty)
           unit.error(clazz.pos, abstractErrorMessage)
+      } else if (clazz.isTrait) {
+        // prevent abstract methods in interfaces that override final members in Object; see #4431
+        for (decl <- clazz.info.decls.iterator) {
+          val overridden = decl.overriddenSymbol(ObjectClass)
+          if (overridden.isFinal)
+            unit.error(decl.pos, "trait cannot redefine final method from class AnyRef")
+        }
       }
 
       /** Returns whether there is a symbol declared in class `inclazz`
diff --git a/test/files/neg/t4431.check b/test/files/neg/t4431.check
new file mode 100644
index 0000000000..7896ec1a62
--- /dev/null
+++ b/test/files/neg/t4431.check
@@ -0,0 +1,7 @@
+t4431.scala:5: error: class BB needs to be abstract, since there is a deferred declaration of method f which is not implemented in a subclass
+  class BB extends B { def f (): Unit }
+        ^
+t4431.scala:8: error: trait cannot redefine final method from class AnyRef
+  trait C { def wait (): Unit }
+                ^
+two errors found
diff --git a/test/files/neg/t4431.scala b/test/files/neg/t4431.scala
new file mode 100644
index 0000000000..5fbb239e04
--- /dev/null
+++ b/test/files/neg/t4431.scala
@@ -0,0 +1,16 @@
+object Test {
+  // this works.
+  class B { final def f(): Unit = () }
+  trait A extends B { def f (): Unit }
+  class BB extends B { def f (): Unit }
+
+  // this earns a VerifyError.
+  trait C { def wait (): Unit }
+  class D { }
+
+  def main(args: Array[String]): Unit = {
+    new B with A { }
+    new BB
+//    new D with C { }
+  }
+}