Some sources cannot be compiles with --release 16, as some public, exported, classes extend internal classes that are not available iin the ct.sym. Examples include:
---
import jdk.jfr.Event;

public class EventTest {
    private void t(Event evt) {
        evt.isEnabled();
    }
}
---
$ javac --release 16 /tmp/EventTest.java
/tmp/EventTest.java:5: error: cannot access Event
        evt.isEnabled();
           ^
  class file for jdk.internal.event.Event not found
1 error
---
public class VectorTest {
     {
         jdk.incubator.vector.Vector v = null;
         v.toString();
     }
}
---
$ javac --release 16 --add-modules jdk.incubator.vector /tmp/VectorTest.java
warning: using incubating module(s): jdk.incubator.vector
/tmp/VectorTest.java:4: error: cannot access VectorSupport
         v.toString();
          ^
  class file for jdk.internal.vm.vector.VectorSupport not found
1 error
1 warning
---