union_hack.h

struct empty_struct {};

struct dummy_type {
  static_assert(sizeof(T) % sizeof(empty_struct) == 0, "");
  // Use an array to avoid GCC 6 placement-new warning.
  empty_struct data[sizeof(T) / sizeof(empty_struct)];
};

// Data storage
union {
  T data_;
  dummy_type dummy_;
};