Variant cases with concepts

cases.cpp

#include <iostream>
#include <vector>
#include <variant>
#include <string>

using namespace std;

template<typename ... Ts>
struct cases : Ts ... {using Ts::operator() ...; };
template<class... Ts> cases(Ts...) -> cases<Ts...>;

template<typename T>
concept printable = requires(T a) {
    { cout << a } -> same_as<ostream&>;
};

struct complex { double re, im; };
ostream& operator<<(ostream& os, complex c) {
    return os << c.re << " + " << c.im << "i";
}

int main(){
    using types = variant<string, long, int, long long, complex>;
    vector<types> vecVariant = {"hello", '2', 100ll, 2011l, 2017, complex{2, 1}};

    for (auto v : vecVariant) {
        visit( cases {
        [](string) { cout << "string" << endl; },
        [](int) { cout << "int" << endl; },
        [](unsigned int) { cout << "unsigned int" << endl; },
        [](long int) { cout << "long int" << endl; },
        [](long long int) { cout << "long long int" << endl; },
        [](printable auto i) { cout << "unknown type (" << i << ")" << endl; }
        }, v);
    };
}