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) {

  }

  static @Nullable 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) {

   * errors for callers like {@link TypeToInstanceMap}.
   */
  @CanIgnoreReturnValue
  final TypeToken<T> rejectTypeVariables() {
    new TypeVisitor() {
      @Override
      void visitTypeVariable(TypeVariable<?> type) {
        throw new IllegalArgumentException(
            runtimeType + "contains a type variable and is not safe for the operation");
      }

      @Override