Java 收集器到Optional

使用Java Streams和泛型，我试图创建一个Collector，如果stream为空，则返回Optional.empty，如果stream有一个项，则返回Optional<T>，如果stream有多个项，则抛出异常.

public static <T, R extends Optional<T>> Collector<T, ?, R> toOptional() {
    return Collectors.collectingAndThen(
            Collectors.toList(),
            list -> {
                if (list.size() > 1) {
                    throw new CollectingException("More than one item found in: " + list);
                }
                if (list.size() == 1) {
                    return Optional.of(list.get(0));
                }
                return Optional.empty();
            }
    );
}

然而，我收到了一个我不理解的错误.

Required type:
Collector<T,?,R>
Provided:
Collector<Object,Object,Object>
no instance(s) of type variable(s) T exist so that Optional<T> conforms to R inference variable RR has incompatible bounds: equality constraints: R lower bounds: Optional<T1868063>

所需的用法将是

List<Person> people = new ArrayList<>();
Optional<Person> john = people.stream().filter(person->"John".equals(person.getName())).collect(toOptional());

有谁能给我解释一下出什么事了吗？