Invalid free when aligned_union is std::swap'd containing small std::string.

aligned_union.cpp

#include <string>
#include <type_traits>
#include <utility>

struct my_empty { };

using storage = std::aligned_union_t<sizeof(my_empty), my_empty, std::string>;

int main()
{
  storage x, y;
  void* x_ptr = &x;
  void* y_ptr = &y;

  new (x_ptr) std::string("foo");
  std::swap(x, y);

  using T = std::string;
  reinterpret_cast<T*>(y_ptr)->~T();
}

http://stackoverflow.com/questions/36965533/why-doesnt-stdswap-work-on-stdaligned-union