问为什么具有类内初始化的类不能用作联合的成员？

EN

#include <iostream>
using namespace std;

struct S{
    int a = 0;
    S() = default;
};

union U {
    S s;
    int i;
};

int main() {
    U u;
    u.s.a = 1;
    return 0;
}

prog.cpp: In function ‘int main()’:
prog.cpp:15:4: error: use of deleted function ‘U::U()’
  U u;
    ^
prog.cpp:9:7: note: ‘U::U()’ is implicitly deleted because the default definition would be ill-formed:
 union U {
       ^
prog.cpp:10:4: error: union member ‘U::s’ with non-trivial ‘constexpr S::S()’
  S s;
    ^

union U {
    S s;
    int i;
    U() {
 
    }
};