broken error message for subclass of interface with private method

FULL PRODUCT VERSION :
java version "9.0.1"
Java(TM) SE Runtime Environment (build 9.0.1+11)
Java HotSpot(TM) 64-Bit Server VM (build 9.0.1+11, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
17.2.0 Darwin Kernel Version 17.2.0: Fri Sep 29 18:27:05 PDT 2017; root:xnu-4570.20.62~3/RELEASE_X86_64 x86_64


A DESCRIPTION OF THE PROBLEM :
The following code is quite confusing, as after casting an anonymous class to its interface it can work as a functional interface (in various constructions actually):

public class A {
    public static void main(String[] args) {
        Runnable test1 = ((I)(new I() {}))::test; // compiles OK
        Runnable test2 = ((new I() {}))::test; // won't compile
    }

    interface I {
        private void test() {}
    }
}

So, Runnable test1 = ((I)(new I() {}))::test; compiles and works okay, while Runnable test2 = ((new I() {}))::test; doesn't compile.

It seems that it is not a consistent behavior. Either one should be compiled well and another, or both shouldn't be compiled.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the following code:

public class A {

    public static void main(String[] args) {
        Runnable test1 = ((I)(new I() {}))::test;
        Runnable test2 = ((new I() {}))::test;
    }

    interface I {
        private void test() {}
    }

}


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Perhaps, Runnable test2 = ((new I() {}))::test; should be compiled fine in this example.
ACTUAL -
Runnable test1 = ((I)(new I() {}))::test; is compiled well
Runnable test2 = ((new I() {}))::test; is not compiled.



REPRODUCIBILITY :
This bug can be reproduced always.