静态 constexpr int 与老式枚举:何时以及为什么?

This is maybe a basic question, but I cannot see the response by myself right now.

考虑以下代码:

template<bool b>
struct T {
    static constexpr int value = (b ? 42 : 0);
};

template<bool b>
struct U {
    enum { value = (b ? 42 : 0) };
};

int main() {
    static_assert(T<true>::value == 42, "!");
    static_assert(T<false>::value == 0, "!");
    static_assert(U<true>::value == 42, "!");
    static_assert(U<false>::value == 0, "!");
}

我习惯于使用像 T 这样的结构，但我不止一次看到像 U 这样的结构用于相同的目的(主要是特征定义).

#include <iostream>

struct T {
    static constexpr int i = 42;
    enum : int {x = 42};
};

void check(const int& z) {
    std::cout << "Check: " << z << "
";
}

int main() {
    // check(T::i); // Uncommenting this will lead to link error
    check(T::x);
}