#include <iostream>
#include <utility>

template <typename T>
concept Printable = requires(const T &x) {
    { std::cout << x } -> std::same_as<std::ostream &>;
};

template <Printable T1, Printable T2>
std::ostream &operator<<(std::ostream &os, const std::pair<T1, T2> &x) {
    return os << "(" << x.first << ", " << x.second << ")";
}

static_assert(Printable<std::pair<int, int>>); // fails because 'std::pair<int, int>' does not satisfy 'Printable'

A concept Printable indicates that a type can be printed to standard output. std::pair<T1, T2> is expected to be Printable whenever T1 and T2 are both Printable.

However, static_assert(Printable<std::pair<int, int>>) fails. Why and how do I fix it?

More precisely, what I want to do is:

#include <iostream>
#include <utility>
#include <vector>

template <typename T>
concept Printable = requires(const T &x) {
    { std::cout << x } -> std::same_as<std::ostream &>;
};

template <Printable T1, Printable T2>
std::ostream &operator<<(std::ostream &os, const std::pair<T1, T2> &x) {
    return os << "(" << x.first << ", " << x.second << ")";
}

template <Printable T>
std::ostream &operator<<(std::ostream &os, const std::vector<T> &v) {
    for (const auto &e : v) os << e;
    return os;
}

// I want these assertions all to succeed.
static_assert(Printable<std::pair<int, std::vector<int>>>);
static_assert(Printable<std::vector<std::pair<int, int>>>);
static_assert(Printable<std::vector<std::vector<std::vector<int>>>>);