Summary
-------

Add support for _sealed classes_ in javax.lang.model. Sealed classes are classes
or interfaces that restrict which other classes or interfaces may extend or
implement them.

Problem
-------

Sealed classes, see ([JEP 360](http://openjdk.java.net/jeps/360)), will be
previewed in Java SE 15 and support will be needed for them in `javax.lang.model`.
In particular new modifiers will be needed, corresponding to the new `sealed`
and `non-sealed` class modifiers. In addition, it will be necessary to provide a
way to access the permitted subclasses of a sealed class or interface.

Solution
--------

Enhance `javax.lang.model` to support sealed classes as follows:

-  Add two new modifiers to
`javax.lang.model.element.Modifier` representing the `sealed` and 
the `non-sealed` class modifiers. 

- Add a new method to
`javax.lang.model.element.TypeElement` which returns a list containing the
permitted subclasses. This list can only be empty if the given `TypeElement` is not
sealed. 

Specification
-------------

    diff --git a/src/java.compiler/share/classes/javax/lang/model/element/Modifier.java b/src/java.compiler/share/classes/javax/lang/model/element/Modifier.java
    --- a/src/java.compiler/share/classes/javax/lang/model/element/Modifier.java
    +++ b/src/java.compiler/share/classes/javax/lang/model/element/Modifier.java
    @@ -59,6 +59,36 @@ public enum Modifier {
          */
          DEFAULT,
         /** The modifier {@code static} */          STATIC,
    +
    +    /**
    +     * {@preview Associated with sealed classes, a preview feature of the Java language.
    +     *
    +     *           This enum constant is associated with <i>sealed classes</i>, a preview
    +     *           feature of the Java language. Preview features
    +     *           may be removed in a future release, or upgraded to permanent
    +     *           features of the Java language.}
    +     *
    +     * The modifier {@code sealed}
    +     * @since 15
    +     */
    +    @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.SEALED_CLASSES,
    +                                             essentialAPI=false)
    +    SEALED,
    +
    +    /**
    +     * {@preview Associated with sealed classes, a preview feature of the Java language.
    +     *
    +     *           This enum constant is associated with <i>sealed classes</i>, a preview
    +     *           feature of the Java language. Preview features
    +     *           may be removed in a future release, or upgraded to permanent
    +     *           features of the Java language.}
    +     *
    +     * The modifier {@code non-sealed}
    +     * @since 15
    +     */
    +    @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.SEALED_CLASSES,
    +            essentialAPI=false)
    +    NON_SEALED,
         /** The modifier {@code final} */           FINAL,
         /** The modifier {@code transient} */       TRANSIENT,
         /** The modifier {@code volatile} */        VOLATILE,
    diff --git a/src/java.compiler/share/classes/javax/lang/model/element/TypeElement.java b/src/java.compiler/share/classes/javax/lang/model/element/TypeElement.java
    --- a/src/java.compiler/share/classes/javax/lang/model/element/TypeElement.java
    +++ b/src/java.compiler/share/classes/javax/lang/model/element/TypeElement.java
    @@ -204,6 +204,29 @@ public interface TypeElement extends Element, Parameterizable, QualifiedNameable
             return List.of();
         }
     
    +    /**
    +     * {@preview Associated with sealed classes, a preview feature of the Java language.
    +     *
    +     *           This method is associated with <i>sealed classes</i>, a preview
    +     *           feature of the Java language. Preview features
    +     *           may be removed in a future release, or upgraded to permanent
    +     *           features of the Java language.}
    +     * Returns the permitted subclasses of this type element in
    +     * declaration order.
    +     *
    +     * @implSpec The default implementations of this method returns an
    +     * empty and unmodifiable list.
    +     *
    +     * @return the permitted subclasses, or an empty list if there are none
    +     *
    +     * @since 15
    +     */
    +    @jdk.internal.PreviewFeature(feature=jdk.internal.PreviewFeature.Feature.SEALED_CLASSES,
    +                                 essentialAPI=false)
    +    default List<? extends TypeMirror> getPermittedSubclasses() {
    +        return List.of();
    +    }
    +
         /**
          * Returns the package of a top-level type and returns the
          * immediately lexically enclosing element for a {@linkplain

Additional links
-------

* javadoc: http://cr.openjdk.java.net/~vromero/sealed_CSRs/javax.lang.model/javadoc.00/module-summary.html
* specdiff: http://cr.openjdk.java.net/~vromero/sealed_CSRs/javax.lang.model/specdiff.00/overview-summary.html