From:
http://mail.openjdk.java.net/pipermail/jigsaw-dev/2017-February/011337.html

---

src/moda/module-info.java:

module moda {
     exports pkga;
}

src/moda/pkga/Outer.java:

package pkga;
class Outer {
     public static class Untouchable {
         public static void touch() {
             throw new AssertionError("Can't touch this!");
         }
     }
}

src/modb/module-info.java:

module modb {
     requires moda;
}

src/modb/pkgb/Intruder.java:

package pkgb;
public class Intruder {
     public static void main(String[] args) {
         pkga.Outer.Untouchable.touch();
     }
}

$ javac -d out --module-path out --module-source-path src `find src
-name '*.java'`
src/modb/pkgb/Intruder.java:6: error: package pkga is not visible
         pkga.Outer.Untouchable.touch();
         ^
   (package pkga is declared in module moda, which does not export it to
module modb)
1 error

The error is confusing and incorrect.