template <class R, class... ArgTypes>
void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;
Effects: x.swap(y)
template <class R, class Alloc>
struct uses_allocator<packaged_task<R>, Alloc>
: true_type { };
Requires: Alloc shall be an Allocator ([allocator.requirements]).