为什么{}工作而while（）不在这里初始化原子对象？

Question

考虑代码：

#include <atomic>

struct stru {
  int a{};
  int b{};
};

int main() {
  // Doesn't work: std::atomic<stru> as({});
  std::atomic<stru> as{{}};
}

为直接初始化生成的错误消息如下：

prog.cc: In function 'int main()':
prog.cc:9:26: error: call of overloaded 'atomic(<brace-enclosed initializer list>)' is ambiguous
   std::atomic<stru> as({});
                          ^
In file included from prog.cc:1:0:
/opt/wandbox/gcc-7.2.0/include/c++/7.2.0/atomic:200:17: note: candidate: constexpr std::atomic<_Tp>::atomic(_Tp) [with _Tp = stru]
       constexpr atomic(_Tp __i) noexcept : _M_i(__i) { }
                 ^~~~~~
/opt/wandbox/gcc-7.2.0/include/c++/7.2.0/atomic:196:7: note: candidate: std::atomic<_Tp>::atomic(const std::atomic<_Tp>&) [with _Tp = stru] <deleted>
       atomic(const atomic&) = delete;
   ^~~~~~

我认为在这种特殊情况下，这两种形式应该没有区别。为什么会这样？