Why does the java compiler creates a synthetic constructor in outer class?

I can't understand oracle java tutorial that writes:

public class SyntheticConstructor {
    private SyntheticConstructor() {}
    class Inner {
    // Compiler will generate a synthetic constructor since
    // SyntheticConstructor() is private.
    Inner() { new SyntheticConstructor(); }
    }
}

Since the inner class's constructor references the private constructor of the enclosing class, the compiler must generate a package-private constructor.