การกำหนด Concepts ด้วย requires

template <typename T>
concept Integer = std::is_integral_v<T>;

template <typename T>
concept SignedInteger = std::is_integral_v<T> && std::is_signed_v<T>;

template <typename T>
concept Addable = requires(T a, T b) {
    a + b;
};

template <typename T>
concept Number = requires(T a, T b) {
    { a + b } -> std::convertible_to<T>;
    { a * b } -> std::convertible_to<T>;
};

template <typename T>
concept Sortable = requires(T t) {
    { t.begin() };
    { t.end() };
    requires std::movable<typename T::value_type>;
};

template <typename T>
concept HasValueType = requires {
    typename T::value_type;
};

// 1. Replace typename
template <Integer T>
T increment(T x) { return x + 1; }

// 2. requires after template parameters
template <typename T>
    requires Integer<T>
T increment(T x) { return x + 1; }

// 3. requires after function signature
template <typename T>
T increment(T x) requires Integer<T> {
    return x + 1;
}

void process(Integer auto x) { std::cout << x; }

template <std::integral T, std::integral U>
    requires (sizeof(T) >= sizeof(U))
T promote(U value) { return static_cast<T>(value); }

template <std::integral T>
class Counter { T value_; /* ... */ };