When compiling a class that imports a class referenced from a Jar, that in turn extends a class to be generated during annotation processing, a NullPointerException can occur.

Steps to reproduce:

A.java:
package t1;

public @interface A {
}

B.java:
package t2;

public class B {
}

C.java:
package t3;

import t2.B;

public class C extends B {
}

D.java:
import t1.A;
import t3.C;
import t2.B;

@A
public class D {
}

P.java:
import java.io.IOException;
import java.io.Writer;
import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.ElementFilter;
import javax.tools.Diagnostic;
import javax.tools.JavaFileObject;

@SupportedAnnotationTypes("t1.A")
public class P extends AbstractProcessor {

    private static final String B_JAVA = "package t2; public class B {}";

    @Override
    public final boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (TypeElement te : ElementFilter.typesIn(roundEnv.getRootElements())) {
            createFile(te);
        }
        return true;
    }

    private void createFile(TypeElement te) {
        // Generate B.java when the processor reads the @A annotation on D
        if ("D".equals(te.getSimpleName().toString())) {
            try {
                JavaFileObject fo = processingEnv.getFiler().createSourceFile("B");
                try (Writer out = fo.openWriter()) {
                    out.write(B_JAVA);
                }
            } catch (IOException e) {
                processingEnv.getMessager().printMessage(Diagnostic.Kind.ERROR, "problem writing file: " + e);
            }
        }
    }

    @Override
    public SourceVersion getSupportedSourceVersion() {
        return SourceVersion.latestSupported();
    }
}

Compile processor:
javac P.java t1/A.java t2/B.java t3/C.java

Create Jar containing C:
jar -cf t3.jar t3/C.class

Delete B:
rm t2/B.*

Compile D with processor P and observe NullPointerException:
javac -processor P D.java
An exception has occurred in the compiler (16-internal). Please file a bug against the Java compiler via the Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.java.com) for duplicates. Include your program, the following diagnostic, and the parameters passed to the Java compiler in your report. Thank you.
java.lang.NullPointerException: Cannot read field "kind" because "tree.sym" is null
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitIdent(Flow.java:2756)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCIdent.accept(JCTree.java:2407)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scanExpr(Flow.java:1921)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitApply(Flow.java:2623)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodInvocation.accept(JCTree.java:1745)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.visitExec(TreeScanner.java:219)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCExpressionStatement.accept(JCTree.java:1532)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:57)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitBlock(Flow.java:2197)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCBlock.accept(JCTree.java:1047)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitMethodDef(Flow.java:2100)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCMethodDecl.accept(JCTree.java:893)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.visitClassDef(Flow.java:2038)
at jdk.compiler/com.sun.tools.javac.tree.JCTree$JCClassDecl.accept(JCTree.java:790)
at jdk.compiler/com.sun.tools.javac.tree.TreeScanner.scan(TreeScanner.java:49)
at jdk.compiler/com.sun.tools.javac.comp.Flow$BaseAnalyzer.scan(Flow.java:440)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.scan(Flow.java:1664)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.analyzeTree(Flow.java:2806)
at jdk.compiler/com.sun.tools.javac.comp.Flow$AssignAnalyzer.analyzeTree(Flow.java:2788)
at jdk.compiler/com.sun.tools.javac.comp.Flow.analyzeTree(Flow.java:217)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1404)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.flow(JavaCompiler.java:1378)
at jdk.compiler/com.sun.tools.javac.main.JavaCompiler.compile(JavaCompiler.java:973)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:317)
at jdk.compiler/com.sun.tools.javac.main.Main.compile(Main.java:176)
at jdk.compiler/com.sun.tools.javac.Main.compile(Main.java:64)
at jdk.compiler/com.sun.tools.javac.Main.main(Main.java:50)
printing javac parameters to: /tmp/javac.20201120_161206.args