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

Jonathan Wakely jwakely@redhat.com
Thu Jun 10 10:43:39 GMT 2021


On Thu, 10 Jun 2021 at 11:34, Jakub Jelinek wrote:
>
> 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

Those people can write their own pure-C++23 library then ;-)

> can't use it for C++20 (due to the -Wpedantic warnings or -pedantic-errors

Yeah.

> errors) and for C++17 and earlier (consteval is not a keyword).

std::is_constant_evaluated isn't defined for C++17 and earlier, so in
C++14 and C++17 code we have to use the builtin directly (and for
C++11 constexpr is so limited that there are no uses for it).

I think we should just leave it as-is, and revisit if a good reason
arises in future.



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