[libcxx-dev] Sequence container swap noexcept specifiers not-to-spec

[Louis Dionne via libcxx-dev]

> On Feb 20, 2021, at 14:12, Christopher Di Bella <cjdb.ns at gmail.com> wrote:
> 
> I've noticed that our sequence containers' noexcept specifiers for swap aren't to spec. Is this intentional, or some gross oversight?
> 
> Example from <vector>
> 
>     void swap(vector&)
> #if _LIBCPP_STD_VER >= 14
>         _NOEXCEPT;
> #else
>         _NOEXCEPT_(!__alloc_traits::propagate_on_container_swap::value ||
>                     __is_nothrow_swappable<allocator_type>::value);
> #endif
> 
> To me, this looks like it's saying that swap is unconditionally noexcept in C++17 and C++20, and possibly the opposite of [vector.capacity]/12 otherwise?
> 
> [vector.capacity]/12 <http://eel.is/c++draft/vector#capacity-12> notes:
> 
> constexpr void swap(vector& x)
>   noexcept(allocator_traits<Allocator>::propagate_on_container_swap::value ||
>            allocator_traits<Allocator>::is_always_equal::value);
> 
> If this is an oversight, I'll file a bug and patch ASAP. If it's intentional, is it explicitly documented somewhere to explain the deviation? This was a surprise when writing tests for std::ranges::swap's noexcept status.

Based on https://en.cppreference.com/w/cpp/container/vector/swap, it seems to me like what we want here is:

    void swap(vector&)
    #if _LIBCPP_STD_VER < 17
        _NOEXCEPT
    #else
        _NOEXCEPT_((std::allocator_traits<Allocator>::propagate_on_container_swap::value ||
                    std::allocator_traits<Allocator>::is_always_equal::value))
    #endif

In other words, mark it as unconditionally noexcept (as a conforming extension) pre-C++17, and implement the noexcept specification per the spec in C++17 and above. Do you agree? If so, I think it would be great to submit a patch and add tests for all the places where this is broken. I would be verrrrry grateful if you were willing to do that, I’ll review it.

Louis

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