Re: Dropping C++03 support (again)

From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2023-02-20 23:50:25

On 2/21/23 02:01, Steven Robbins via Boost wrote:
> On Monday, February 20, 2023 4:45:33 P.M. CST Andrey Semashev via Boost wrote:
>> On 2/21/23 01:00, Gavin Lambert via Boost wrote:
>>> On 21/02/2023 00:30, Andrey Semashev wrote:
>>>> I think you're missing the point that operator== isn't always used to
>>>> implement equivalence. This is also true wrt. other operators and their
>>>> default function. We have a number of lambda libraries in Boost, as well
>>>> as Boost.Proto and Boost.Spirit that wouldn't have existed if we
>>>> followed your logic.
>>>
>>> No, I addressed that.  Even in lambda libraries, operator== should be
>>> used only for equivalence (as an indirection rather than directly), and
>>> as such it should still be commutative and agnostic of specific order.
>>> Assuming that, it shouldn't be broken by the C++20 changes.
>>
>> In template expressions, operators are generally not commutative because
>> the type of the result depends on types of the left and right arguments.
> 
> Isn't the result of "==" always boolean?

The result can have any type. However, the rewritten candidates for
operator== are only generated when the result is bool, so expression
templates should be unaffected by this operator rewriting. I'm not sure
about other comparison operators, though.