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 
struct is_std_container : std::false_type { };

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

And then just SFINAE on that:

template 
typename std::enable_if<
    is_std_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()) == std::declval())
>