tmp/test.scala:4: error overriding class Foo.I in class Foo;
 class Bar.I in class Bar needs `override' modifier
class Bar extends Foo { class I; }
                              ^
one error found


 After adding the missing override modifier, we get the following error message: 

tmp/test.scala:2: `override' modifier not allowed for classes
class Bar extends Foo { override class I; }
                                       ^
one error found

In both case, the error message should say that classes may not be overridden.