std::forward_list::swap to use propagate_on_container_swap for noexcept specification

[airglow923]

Summary

This revision is a patch for bug 50224.

The current implementation of std::forward_list::swap uses propagate_on_container_move_assignment for noexcept specification. This revision has been made in an attempt to change it to propagate_on_container_swap.

[• Quuxplusone]

This does actually look like a pretty straightforward typo in forward_list; the other containers all seem to get it right.
It's too bad that none of our tests notice the change; but, the POCCA/POCMA/POCS traits are obscure, and this affects literally nothing but -std=c++11 mode. I don't think it makes sense to hold up this patch for lack of tests.
(@airglow923 please wait for an approval from "libc++" before landing. @ldionne ping! am I missing anything subtle here?)

[ldionne]

Thanks a lot for fixing this and sorry it went under the radar.

libcxx/include/forward_list

void __move_assign_alloc(__forward_list_base& __x)
               __node_traits::propagate_on_container_move_assignment::value>());}
 
 public:
     _LIBCPP_INLINE_VISIBILITY
     void swap(__forward_list_base& __x)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT;
 #else
-        _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value ||
+        _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<__node_allocator>::value);
 #endif
 protected:
     void clear() _NOEXCEPT;
 
 private:
     _LIBCPP_INLINE_VISIBILITY
     void __copy_assign_alloc(const __forward_list_base&, false_type) {}

 
 template <class _Tp, class _Alloc>
 inline
 void
 __forward_list_base<_Tp, _Alloc>::swap(__forward_list_base& __x)
 #if _LIBCPP_STD_VER >= 14
         _NOEXCEPT
 #else
-        _NOEXCEPT_(!__node_traits::propagate_on_container_move_assignment::value ||
+        _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value ||
                     __is_nothrow_swappable<__node_allocator>::value)
 #endif
 {
     _VSTD::__swap_allocator(__alloc(), __x.__alloc(),
             integral_constant<bool, __node_traits::propagate_on_container_swap::value>());
     using _VSTD::swap;
     swap(__before_begin()->__next_, __x.__before_begin()->__next_);
 }