Re: [release] 1.74.0 post-beta merges

From: Marc Glisse (marc.glisse_at_[hidden])
Date: 2020-07-22 17:06:39

On Wed, 22 Jul 2020, Niall Douglas via Boost wrote:

> On 22/07/2020 17:39, Andrey Semashev via Boost wrote:
>> On 2020-07-22 19:25, Niall Douglas via Boost wrote:
>>>
>>> So Peter, seeing as you're the closest person to an expert in C++ 20
>>> operator rewriting that I know of (and there is absolutely nothing about
>>> this on the internet),
>>
>> FWIW, I found this article helpful to understand the new operators
>> behavior:
>>
>> https://brevzin.github.io/c++/2019/07/28/comparisons-cpp20/
>>
>> TL;DR version is in the summary:
>>
>> https://brevzin.github.io/c++/2019/07/28/comparisons-cpp20/#summary-of-rules
>
> That was very useful.
>
> It turns out that for operator== only, C++ 20 will auto reverse it. So,
> in Outcome, I need to disable free function reversed operator== only,
> for C++ 20 and later. I must leave free function reversed operator!= alone.

Because operator== rewriting broke quite a bit of code, some committee 
people are trying to revert this change (make it opt-in or something) via 
DR. So it may be safer for now to write code in a way that works for both 
C++17 and C++20 rules without #ifdefs, where possible (or fully use <=> 
where that makes sense, which counts as opt-in).

-- 
Marc Glisse