Re: improve <boost/config.hpp>

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2023-03-03 14:39:22

> But I don't understand why you are against it. After all, there is 
> BOOST_CXX14_CONSTEXPR (BOOST_CXX17/20_CONSTEXPR are still missing - 
> but I already offered that). So we have an expressive scheme 
> BOOST_CXXnn_CONSTEXPR that is unambiguous and readable. 

C++14 lifted some major constraints on what can be in a constexpr 
function compared to C++11. Most notably support for more than just a 
return statement. Being able to differentiate that is useful in practice.

 > For example, std::round in C++23 is constexpr

The CXXyy part in the Boost.Config macros refers to the language level 
support. While std::round is at library level. So a compiler supporting 
C++14 constexpr syntax might not have a std library where std::round is 
constexpr even if it claims C++23 support. So the better way here would 
be a more focused macro combining the language level of the constexpr 
feature and the library support of what that specific function is using.

Hence a possible BOOST_CXX23_CONSTEXPR is misleading: It doesn't refer 
to the language level anymore. And if you bind that to availability of 
constexpr std::round the next user may want to use it with another std:: 
function which is supposed to be constexpr in C++23 but due to a std 
library defect is not yet. Hence the distinction between library level 
and language level feature macros.

However IIRC some C++ standard added support of (temporary) allocations 
in constexpr functions. That would be worth a dedicated feature macro as 
that change fits the idea described above.

I hope that helps,
Alex