*     visit(t.getOwnerType());
 *     visit(t.getActualTypeArguments());
 *   }
 *   protected void visitGenericArrayType(GenericArrayType t) {
 *     visit(t.getGenericComponentType());
 *   }
 *   protected void visitTypeVariable(TypeVariable<?> t) {
 *     throw new IllegalArgumentException("Cannot contain type variable.");
 *   }
 *   protected void visitWildcardType(WildcardType t) {

    }.visit(String.class);
  }

  public <T> void testVisitTypeVariable() {
    Type type = new TypeCapture<T>() {}.capture();
    assertVisited(type);
    new BaseTypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> t) {}
    }.visit(type);
  }

  public void testVisitWildcardType() {
    WildcardType type = Types.subtypeOf(String.class);
    assertVisited(type);
    new BaseTypeVisitor() {

      Map<TypeVariableKey, Type> mappings, Type from, Type to) {
    if (from.equals(to)) {
      return;
    }
    new TypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> typeVariable) {
        mappings.put(new TypeVariableKey(typeVariable), to);
      }

      @Override
      void visitWildcardType(WildcardType fromWildcardType) {

      Map<TypeVariableKey, Type> mappings, Type from, Type to) {
    if (from.equals(to)) {
      return;
    }
    new TypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> typeVariable) {
        mappings.put(new TypeVariableKey(typeVariable), to);
      }

      @Override
      void visitWildcardType(WildcardType fromWildcardType) {

  @CheckForNull
  static Type getComponentType(Type type) {
    checkNotNull(type);
    AtomicReference<@Nullable Type> result = new AtomicReference<>();
    new TypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> t) {
        result.set(subtypeOfComponentType(t.getBounds()));
      }

      @Override
      void visitWildcardType(WildcardType t) {