The following program, when compiled on JDK9 dev tip:

interface Foo<X> {
   void g(X x);
}

class Test2 {
   void m() {
      Foo<String> fs = new Foo<>() {
          public void g(String s) { }
          void someMethod() { }
      };
   }
}

elicits the error:

/home/srikanth/tmp/X.java:9: error: method does not override or implement a method from a supertype
          void someMethod() { }
               ^
1 error

This is intentional.

There are some cases with diamond inference where the inferred types are
not what would be "obvious" choices from the p.o.v of a non-discerning user. So a programmer may end up writing some methods that
seemingly override a super type method but don't in reality. To help
diagnose the situation, the spec requires that all non-private instance
methods of a <> inferred anonymous class be treated as though they were
annotated with @Override (See https://bugs.openjdk.java.net/browse/JDK-8073593)

However, the message emitted as of now does not make this explicit and so is a potential grounds for confusion (and invalid defect reports)

javac should emit a clearer message.