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

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jun 10, 2021 at 11:24:43AM +0100, Jonathan Wakely wrote:
> > 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?)

I guess compile-time lost in the noise.

> 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.

Up to you.  The wording says Equivalent to
if consteval { return true; } else { return false; }
and return __builtin_is_constant_evaluated (); is equivalent to that.

Perhaps some people could appreciate to see it literally there, but we
can't use it for C++20 (due to the -Wpedantic warnings or -pedantic-errors
errors) and for C++17 and earlier (consteval is not a keyword).

	Jakub