Map-to-interface cast: map method definition does not override the default method

import groovy.lang.GroovyShell;
import org.codehaus.groovy.control.CompilerConfiguration;

public class InterfaceDefaultMethods {

    public static void main(String... args) throws Exception {
        final CompilerConfiguration cc = new CompilerConfiguration();
        cc.setTargetBytecode(CompilerConfiguration.JDK8);
        final GroovyShell shell = new GroovyShell(cc);
        final X x = (X) shell.evaluate("[x : {-> 3}, z: {-> 19}] as " + X.class.getCanonicalName());
        System.out.format("x = %d, y = %d, z = %d%n", x.x(), x.y(), x.z());
    }

    public interface X {

        default int x() {
            return 1;
        }

        default int y() {
            return 2;
        }

        int z();
    }
}