[llvm-bugs] [Bug 50224] New: std::forward_list::swap noexcept specification not using correct member type

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50224

            Bug ID: 50224
           Summary: std::forward_list::swap noexcept specification not
                    using correct member type
           Product: libc++
           Version: 12.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: p.hyundeok76 at gmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

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_);
}
```

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