@Override
    protected Integer defaultAction(TypeMirror e, Set<Element> visiting) {
      return hashKind(HASH_SEED, e);
    }
Returns the type of the innermost enclosing instance, or null if there is none. This is the
same as {@link DeclaredType#getEnclosingType()} except that it returns null rather than NoType
for a static type. We need this because of <a
href="https://bugs.eclipse.org/bugs/show_bug.cgi?id=508222">this bug</a> whereby the Eclipse
compiler returns a value for static classes that is not NoType.
common/src/main/java/com/google/auto/common/MoreTypes.java