Some general suggestion
I would avoid using namespace std which your code implies (almost) that you use.
You can shorten and make more readable std::enable_if<T>::type and std::is_integral<T>::value by writing them as std::enable_if_t<T> and std::is_integral_v<T> respectively (search for "Helper types" here, for instance).
The answer to your question
You need to apply operator[] to something
What you really want to ask is if the items in the object of type V can be accessed via [], so you want to check whether something like V{}[0] is valid, so you would write something like this (as to why std::decay_t is needed, see note (¹)):
template<class V, typename std::enable_if_t<std::is_integral_v<std::decay_t<decltype(V{}[0])>>, int> = false>
But V might be not default constructible
If you want to support types V without assuming that they are default constructible you can use std::declval to "pretend" you create a value of that type (here std::decay_t is needed for the same reason as above):
template<class V, typename std::enable_if_t<std::is_integral_v<std::decay_t<decltype(std::declval<V>()[0])>>, int> = false>
And operator[] could return some proxy to the actual entities in V
In general operator[] doesn't return an object of the type which is "conceptually" the type stored in the container; for instance, for std::vector<bool>, operator[] doesn't return a bool&, but a proxy type²; in those cases, you really want to rely on the value_type type alias stored in the container:
template<class V, typename std::enable_if_t<std::is_integral_v<typename V::value_type>, int> = false>
Something might be not value_type-equipped but only provide .begin()/.end()
As a last improvement, we can also remove the assumption that the V has a value_type member alias (which is true for STL container), and simply require that it has a begin member function (STL container have it too), and make use of ranges::iter_value_t³:
template<class V, typename std::enable_if_t<std::is_integral_v<ranges::iter_value_t<decltype(std::declval<V>().begin())>>, int> = 0>
Notice that
ranges::iter_value_t<decltype(std::declval<V>().begin())>
is in general not the same same thing as
std::decay_t<decltype(*std::declval<V>().begin())>
a case of when they are different being, again, std::vector<bool>.
¹ std::vector<int>::operator[], for instance, returns int&, not int, and int& is not integral:
static_assert(std::is_integral_v<int&>); // fails
² For instance:
// this passes
static_assert(std::is_same_v<int, std::decay_t<decltype(std::declval<std::vector<int>>()[0])>>);
// this doesn't!
static_assert(std::is_same_v<bool, std::decay_t<decltype(std::declval<std::vector<bool>>()[0])>>);
// with my compiler, this one does pass
static_assert(std::is_same_v<std::_Bit_reference, std::decay_t<decltype(std::declval<std::vector<bool>>()[0])>>);
³ ranges::iter_value_t is from the header #include <range/v3/iterator/access.hpp> of the Range-v3 library. The documentation of that library is close to crap, so you can refer the doc of the analagous C++20 functionality on cppreference.
