How to SFINAE out non-containers parameters

Question

I have a template function that I want to enable only for standard containers (or containers compatible with standard containers, which at least provide a begin()

Answer 1

Using void_t, we can just make a type trait for having begin() and end() (and anything else you might want to check for, like typename T::iterator, you can just keep piling expressions on):

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

template <typename T>
struct is_std_container<T,
    void_t<decltype(std::declval<T&>().begin()),
           decltype(std::declval<T&>().end()),
           typename T::value_type
           >>
    : std::true_type { };

And then just SFINAE on that:

template <typename Container>
typename std::enable_if<
    is_std_container<Container>::value,
    typename Container::value_type
>::type 
f(const Container& c) { .. }

Also, if you really wanted to verify that begin() gives you back a T::iterator (or at least that they're equality comparable), you can do that too:

void_t<
    decltype(begin(std::declval<T&>()) == std::declval<typename T::iterator>())
>