Using boost::hana to check for method existence

Question

I'm trying to create a templated function contains that works for any container by trying these two steps:

• Using the container's own find method .find(value) if it exists (which should be optimized for that specific container)
• Using std::find(xs.cbegin(), xs.cend(), v) as a fallback

It seems that boost::hana provides an is_valid or a sfinae function to check for that, but I wasn't able to figure out how they work...

template<typename TContainer, typename T>
bool
contains(
    const T &v,
    const TContainer &container)
{
    constexpr auto has_find = boost::hana::is_valid([](auto &&xs, auto &&x) -> decltype((void)xs.find(x)) {});

    if constexpr(has_find) {
        return container.find(v) != container.cend();
    } else {
        return std::find(container.cbegin(), container.cend(), v) != container.cend();
    }
}