为什么' std: forward d_as_tuple(1) '不是一个常量表达式?

#include <tuple>

int main() {
  static_assert(std::is_same<std::tuple<int&&>,
                             decltype(std::forward_as_tuple(1))>::value, "");
  constexpr int x = 5;
  constexpr auto t1 = std::forward_as_tuple(1);  // (1)
  constexpr auto t2 = std::forward_as_tuple(x);  // (2)
  constexpr std::tuple<int&&> t3(1);             // (3)
  constexpr std::tuple<int> t4(1); // OK!
}
#include <tuple>

int main() {
  static_assert(