How to write a type trait `is_container` or `is_vector`?

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Look, another SFINAE-based solution for detecting STL-like containers:

template<typename T, typename _ = void>
struct is_container : std::false_type {};

template<typename... Ts>
struct is_container_helper {};

template<typename T>
struct is_container<
        T,
        std::conditional_t<
            false,
            is_container_helper<
                typename T::value_type,
                typename T::size_type,
                typename T::allocator_type,
                typename T::iterator,
                typename T::const_iterator,
                decltype(std::declval<T>().size()),
                decltype(std::declval<T>().begin()),
                decltype(std::declval<T>().end()),
                decltype(std::declval<T>().cbegin()),
                decltype(std::declval<T>().cend())
                >,
            void
            >
        > : public std::true_type {};

Of course, you might change methods and types to be checked.

If you want to detect only STL containers (it means std::vector, std::list, etc) you should do something like this.

UPDATE. As @Deduplicator noted, container might not meet AllocatorAwareContainer requirements (e.g.: std::array<T, N>). That is why check on T::allocator_type is not neccessary. But you may check any/all Container requirements in a similar way.

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