public static WildcardType subtypeOf(Type bound) {
        return new WildcardTypeImpl(
                bound instanceof WildcardType
                        ? ((WildcardType) bound).getUpperBounds()
                        : new Type[]{bound},
                EMPTY_TYPE_ARRAY);
    }
Returns a type that represents an unknown type that extends {@code bound}.
For example, if {@code bound} is {@code CharSequence.class}, this returns
{@code ? extends CharSequence}. If {@code bound} is {@code Object.class},
this returns {@code ?}, which is shorthand for {@code ? extends Object}.
core/src/main/java/com/alibaba/fastjson2/util/BeanUtils.java