#include <cassert>
#include <concepts>
#include <cstddef>
#include <iostream>
#include <vector>

template<class T>
concept HasSize = requires(const T& value) {
    { value.size() } -> std::convertible_to<std::size_t>;
};
template<HasSize T>
std::size_t count(const T& value) { return value.size(); }

int main() {
    static_assert(HasSize<std::vector<int>>);
    static_assert(!HasSize<int>);
    std::vector<int> values{2, 4, 6};
    assert(count(values) == 3);
    std::cout << count(values) << '\n';
}