The current implementation of `std::forward_list::swap` is as follows:

```cpp
_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 ||
                    __is_nothrow_swappable<__node_allocator>::value);
```

which uses `propagate_on_container_move_assignment` for `noexcept` specification.

This must use `propagate_on_container_swap` for checking according to [[container.requirements.general/8]](http://eel.is/c++draft/container.requirements.general#8).

When the change is applied, the declaration should look like:

```cpp
_LIBCPP_INLINE_VISIBILITY
    void swap(__forward_list_base& __x)
#if _LIBCPP_STD_VER >= 14
        _NOEXCEPT;
#else
        _NOEXCEPT_(!__node_traits::propagate_on_container_swap::value ||
                    __is_nothrow_swappable<__node_allocator>::value);
```

and the definition should be as follows:

```cpp
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_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_);
}
```