After CC in  following code:

public class ElementData<T extends Element> extends Properties {
     private ElementHandle<T> elementHandle;
     private Set<Modifier> modifiers;
     public ElementData(T element) {
        this.elementHandle = ElementHandle.create(element);
        this.modifiers = element.^^

The exception was thrown:

java.lang.AssertionError: isSubClass T
at com.sun.tools.javac.code.Symbol.isSubClass(Symbol.java:280)
at com.sun.tools.javac.comp.Resolve.isProtectedAccessible(Resolve.java:239)
at com.sun.tools.javac.comp.Resolve.isAccessible(Resolve.java:207)
at com.sun.tools.javac.comp.Resolve.collectMembers(Resolve.java:450)
at com.sun.tools.javac.comp.Resolve.collectMembers(Resolve.java:479)
at com.sun.tools.javac.comp.Resolve.findMembersOf(Resolve.java:435)
at
org.netbeans.api.java.source.SourceUtils$EnvironmentImpl.getMembersOf(SourceUtils.java:729)
at
org.netbeans.modules.editor.java.JavaCompletionProvider$JavaCompletionTask$JavaCompletionQuery.addMembers(JavaCompletionProvider.java:2447)
at
org.netbeans.modules.editor.java.JavaCompletionProvider$JavaCompletionTask$JavaCompletionQuery.insideMemberSelect(JavaCompletionProvider.java:1529)
at
org.netbeans.modules.editor.java.JavaCompletionProvider$JavaCompletionTask$JavaCompletionQuery.resolveCompletion(JavaCompletionProvider.java:427)
at
org.netbeans.modules.editor.java.JavaCompletionProvider$JavaCompletionTask$JavaCompletionQuery.run(JavaCompletionProvider.java:302)
at
org.netbeans.modules.editor.java.JavaCompletionProvider$JavaCompletionTask$JavaCompletionQuery.run(JavaCompletionProvider.java:158)
at org.netbeans.api.java.source.JavaSource$CompilationJob.run(JavaSource.java:608)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:287)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at
java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:679)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:704)
[catch] at java.lang.Thread.run(Thread.java:619)