JEP 401 (https://openjdk.java.net/jeps/401) states:

// --
An abstract class can similarly be declared to implement either IdentityObject or PrimitiveObject; or, if it declares a field, an instance initializer, a non-empty constructor, or a synchronized method, it implicitly implements IdentityObject (perhaps with a warning). Otherwise, the abstract class extends neither interface and can be extended by both kinds of concrete classes.

// --

So this code should compile:

public abstract class X {
    int f;
    void foo(X x) {
        IdentityObject i = x;
    }
}

but as of now we get:

X.java:4: error: incompatible types: X cannot be converted to IdentityObject
        IdentityObject i = x;
                           ^
1 error