ADDITIONAL SYSTEM INFORMATION :
openjdk version "11.0.2" 2019-01-15
OpenJDK Runtime Environment 18.9 (build 11.0.2+9)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.2+9, mixed mode)

A DESCRIPTION OF THE PROBLEM :
When trying to customize the JavaDoc generation process by wrapping the original DocletEnvironment implementation, the JavaDoc generation fails with a ClassCastException, because the JDK attempts to cast to the "exact" type of the DocletEnvironment instance.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Write a custom doclet that extends the StandardDoclet and uses a delegate for DocletEnvironment and then use the doclet during JavaDoc generation.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The doclet works and no classcastexception is raised.
ACTUAL -
The Workarounds class attempts to cast the class to the default DocEnvImpl type and fails with a ClassCastException.

---------- BEGIN SOURCE ----------
package bug.demo;

import java.util.Set;

import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;
import javax.tools.JavaFileManager;
import javax.tools.JavaFileObject.Kind;

import com.sun.source.util.DocTrees;

import jdk.javadoc.doclet.DocletEnvironment;
import jdk.javadoc.doclet.StandardDoclet;

public class NewDoclet extends StandardDoclet {

    @Override
    public boolean run(DocletEnvironment docEnv) {
        return super.run(wrap(docEnv));
    }

    public DocletEnvironment wrap(DocletEnvironment env) {
        return new DocletEnvironment() {
            @Override
            public Set<? extends Element> getSpecifiedElements() {
                return env.getSpecifiedElements();
            }

            @Override
            public Set<? extends Element> getIncludedElements() {
                return env.getIncludedElements();
            }

            @Override
            public DocTrees getDocTrees() {
                return env.getDocTrees();
            }

            @Override
            public Elements getElementUtils() {
                return env.getElementUtils();
            }

            @Override
            public Types getTypeUtils() {
                return env.getTypeUtils();
            }

            @Override
            public boolean isIncluded(Element element) {
                return env.isIncluded(element);
            }

            @Override
            public boolean isSelected(Element element) {
                return env.isSelected(element);
            }

            @Override
            public JavaFileManager getJavaFileManager() {
                return env.getJavaFileManager();
            }

            @Override
            public SourceVersion getSourceVersion() {
                return env.getSourceVersion();
            }

            @Override
            public ModuleMode getModuleMode() {
                return env.getModuleMode();
            }

            @Override
            public Kind getFileKind(TypeElement type) {
                return env.getFileKind(type);
            }
        };
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Extend the unsupported DocEnvImpl type.

FREQUENCY : always