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