C 11中的工会：默认构造函数似乎被删除

If a union contains a non-static data member with a non-trivial special member function (default constructor,copy/move constructor,copy/move assignment,or destructor),that function is deleted by default in the union and needs to be defined explicitly by the programmer.
At most one data member can have a default member initializer.

我正在尝试以下代码：

struct X
{
    ~X() {};
};

union U
{
    X x;
    ~U() {};
};

int main()
{
    U s1{};  // works,probably aggregate initialization
    U s2;    // DOES NOT compile,why?
}

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这里X(用作联合的数据成员)具有用户提供的析构函数,因此,联合的析构函数默认删除.所以我明确提供一个.但是,代码无法编译,带有错误

note: ‘U::U()’ is implicitly deleted because the default definition would be ill-formed:

如果我删除最后一行U s2 ;.

问题这里发生了什么？为什么U s1 {};编译,但U s2;才不是？是否将联盟的默认ctor标记为已删除(如果是这样,为什么？！),而在第一种情况下,我们只是聚合初始化？请注意,如果我提供U(){}; // not U()= default;代码编译(但不是如果我只提供一个X的ctor).

编辑

挖掘标准(N4527)后：

工会：9.5 / 2 [class.union]

[Note: If any non-static data member of a union has a non-trivial default constructor (12.1),copy constructor (12.8),move constructor (12.8),copy assignment operator (12.8),move assignment operator (12.8),or destructor (12.4),the corresponding member function of the union must be user-provided or it will be implicitly deleted (8.4.3) for the union. —endnote]

似乎这是一个gcc错误(现在报道here).代码编译在clang和gcc 4.8.2或更早版本,它在gcc4.9和更高版本(感谢@ T.C.指出)中断.

编译器：g 5.3,-std = c 11已使用.