Re: is_constant_evaluated

From: John Maddock (jz.maddock_at_[hidden])
Date: 2021-08-11 08:14:12

> I've looked at this and have a couple of questions:
>
> a) I see BOOST_NO_CXX20_IS_CONSTANT_EVALUATED and presume that's it's 
> like all the other boost feature macros.  So far so good.
>
> This might return true or false independently of the C++ version being 
> used.  That is, I could be compiling with C++14 and the macro might 
> return true.  This might direct the generation of the most optimal 
> code but that that code might be different depending on the compiler 
> being tested.  I'm concerned about being able to test for 
> portability/standards conformance.  So I'd like to be able to have 
> BOOST_NO_CXX20_IS_CONSTANT_EVALUATED defined either in accordance with 
> the standard version or with the actual compiler capability.  So I'd 
> run my tests twice:
>
> 1) with the most efficient implementation.
> 2) with the standards conforming implementation.
>
> Assuming it passes 2) (al be it more slowly) I would know that my 
> test/application could always be counted on to pass.
>
> I don't know if I explained myself well.
>
Not really ;)

BOOST_NO_CXX20_IS_CONSTANT_EVALUATED is defined if there is no 
std::is_constant_evaluated() in type_traits: for all current 
compilers/std libs that means real C++20 mode as well as compiler 
intrinsic support.  In contrast BOOST_IS_CONSTANT_EVALUATED() works in 
C++14/17 etc if the required intrinsic is present.

John.

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