Java Visibility Puzzler

Base.java

package sampler;

public class Base {
	void packageMethod() {
		System.err.println("Package on " + getClass());
	}
	
	protected void protectedMethod() {
		System.err.println("Protected on " + getClass());
	}
}

Impl.java

package sampler.internal;

import sampler.Base;

public class Impl extends Base {
  // Comment this in and protected access suddenly fails
  // @Override
  // protected void protectedMethod() { super.protectedMethod(); }
}

User.java

package sampler;

import sampler.internal.Impl;

public class User {
	public static void main(String[] args) {
		Base base = new Base();
		Impl impl = new Impl();
		
		base.packageMethod();
		impl.packageMethod(); // Compile error
		((Base)impl).packageMethod();
		
		base.protectedMethod();
		impl.protectedMethod(); // No compile error
		
		// why can I call a protected method
		// but not a package-private??? I'm confused
	}
}