[libc++] Match declaration for non-member function std::swap(std::packaged_task) with what standard specify

[jasonliu]

Summary

Standard specifies:

template<class R, class... ArgTypes>
  void swap(packaged_task<R(ArgTypes...)>& x, packaged_task<R(ArgTypes...)>& y) noexcept;

[• Quuxplusone]

Hard no. std::swap is not meant to be called with explicit template arguments (and especially not random arguments like <double, int, char>).
Is this an XY problem?

[ldionne]

Agreed with Arthur about the fact that it's not supposed to be called that way. However, the Standard does specify it as being swap(packaged_task<C(A...)>): http://eel.is/c++draft/futures.task#nonmembers-1. So I'm fine with the change as decreasing distance from the Standard, but I think we should *not* add a test.

[jasonliu]

Got it. Make sense. I removed the test case in case we want to have the declaration be more like what standard have in the text.

[ldionne]

@Quuxplusone Do you have a problem with this?

[• Quuxplusone]

Sure; I'm still uncomfortable with making this change to enable the discouraged use-case (even without the test). But there's no real chance anyone would ever ask to change the code back to the old version.

libcxx/include/future

 void
 packaged_task<void(_ArgTypes...)>::reset()
 {
     if (!valid())
         __throw_future_error(future_errc::no_state);
     __p_ = promise<result_type>();
 }
 
-template <class _Callable>
+template <class _Rp, class... _ArgTypes>
 inline _LIBCPP_INLINE_VISIBILITY
 void
-swap(packaged_task<_Callable>& __x, packaged_task<_Callable>& __y) _NOEXCEPT
+swap(packaged_task<_Rp(_ArgTypes...)>& __x, packaged_task<_Rp(_ArgTypes...)>& __y) _NOEXCEPT
 {
     __x.swap(__y);
 }
 
 template <class _Callable, class _Alloc>
 struct _LIBCPP_TEMPLATE_VIS uses_allocator<packaged_task<_Callable>, _Alloc>
     : public true_type {};