Narrow Java exception to inheritance rule

Excepted are only Serializable and Comparable. This follows
scalac's behavior.

Author: odersky

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

@@ -604,8 +604,9 @@ trait Checking {
   /** Given a parent `parent` of a class `cls`, if `parent` is a trait check that
    *  the superclass of `cls` derived from the superclass of `parent`.
    *
-   *  An exception is made if `cls` extends `Any`, and `parent` is a Java class
-   *  that extends `Object`. For instance, we accept code like
+   *  An exception is made if `cls` extends `Any`, and `parent` is `java.io.Serializable`
+   *  or `java.lang.Comparable`. These two classes are treated by Scala as universal
+   *  traits. E.g. the following is OK:
    *
    *      ... extends Any with java.io.Serializable
    *
@@ -617,7 +618,8 @@ trait Checking {
         val psuper = parent.superClass
         val csuper = cls.superClass
         val ok = csuper.derivesFrom(psuper) ||
-          parent.is(JavaDefined) && csuper == defn.AnyClass && psuper == defn.ObjectClass
+          parent.is(JavaDefined) && csuper == defn.AnyClass &&
+          (parent == defn.JavaSerializableClass || parent == defn.ComparableClass)
         if (!ok)
           ctx.error(em"illegal trait inheritance: super$csuper does not derive from $parent's super$psuper", pos)
       case _ =>