Re: [PATCH] c++: Add C++23 consteval if support - P1938R3 [PR100974]

[Jonathan Wakely <jwakely@redhat.com>]

On Thu, 10 Jun 2021 at 09:34, Jakub Jelinek wrote:
> Also, the paper doesn't contain the exact __cpp_if_consteval value,

Right, proposals aren't supposed to, because the value gets set when
the proposal is voted into the working draft (which the proposal
author doesn't control).

> but https://github.com/cplusplus/draft/pull/4660/files mentions 202106L
> which this patch uses.

That's the right value.

> And I'm not changing the libstdc++ side, where perhaps we could change
> std::is_constant_evaluated definition for
> #ifdef __cpp_if_consteval
> case to if consteval { return true; } else { return false; }
> but we need to keep it defined to __builtin_is_constant_evaluated ()
> for C++20 or older.

Is there any advantage to changing that (cheaper for GCC to evaluate?)
or should we just continue to use the __builtin unconditionally?

I suppose it's theoretically possible that there could be a non-GCC
compiler where defined(__cpp_if_consteval) is true but
__has_builtin(__builtin_is_constant_evaluated) is false.