#include <cstddef>
#include <vector>
#include <type_traits>
#include <set>

// C++20

/*
template <typename T>
struct element_type
{
    using type = void;
};

template <typename T>
concept has_value_type = requires(T data)
{
    typename T::value_type;
};

template <has_value_type T>
struct element_type<T>
{
    using type = typename T::value_type;
};
*/

// C++17

template <typename T>
typename T::value_type element_type_helper(int);

template <typename T>
void element_type_helper(double);


template <typename T>
struct element_type
{
    using type = decltype(element_type_helper<T>(0));
};

// Shared between C++20 and C++17

template <typename T>
struct element_type<T*>
{
    using type = T;
};

template <typename T, std::size_t N>
struct element_type<T[N]>
{
    using type = T;
};

int main()
{
    static_assert( std::is_same_v<element_type<float>::type, void> );
    static_assert( std::is_same_v<element_type<double*>::type, double> );
    static_assert( std::is_same_v<element_type<bool[5]>::type, bool> );
    static_assert( std::is_same_v<element_type<std::vector<int>>::type, int> );
    static_assert( std::is_same_v<element_type<std::set<char>>::type, char> );
}