On Fri, 26 Nov 2021 at 12:43, Jonathan Wakely wrote:
>
> On Fri, 26 Nov 2021 at 12:39, Jakub Jelinek wrote:
> >
> > On Fri, Nov 26, 2021 at 12:29:25PM +0000, Jonathan Wakely via Gcc-patches wrote:
> > > +  // Internal version of std::is_constant_evaluated() for C++11.
> > > +  // This can be used without checking if the compiler supports the built-in.
> > > +  constexpr inline bool
> > > +  __is_constant_evaluated() noexcept
> > > +  {
> >
> > When you have such a nice one spot, shouldn't it:
> > #if __cpp_if_consteval >= 202106L
> >   if consteval
> >     {
> >       return true;
> >     }
> >   else
> >     {
> >       return false;
> >     }
> > #elif __has_builtin(__builtin_is_constant_evaluated)
> > ...
> >
> > Theoretically not all compilers need to support the builtin and in C++23
> > mode if consteval should be slightly more efficient.
>
> Yes, good idea. We actually still have two spots, because we still
> have std::is_constant_evaluated as well, which is only defined if it
> actually works. But we can use the same implementation in there (or
> make it call std::__is_constant_evaluated()). I'll prepare a new patch
> soon.
>
> > One disadvantage of std::__is_constant_evaluated() is that Marek's
> > warning for if constexpr (std::is_constant_evaluated()) will not trigger
> > if __is_constant_evaluated() is used instead.  But I'd hope testsuite
> > coverage would discover it quickly if such bug would appear...
>
> Yes, and I hope I won't make that mistake anyway, or miss it in reviews.
>
> It's a good warning for users, but I like to think I don't need it
> (and now we start the sweepstake on how many days until I push a patch
> making exactly that mistake ;-)

Who had -15 days in the sweepstake? r12-5187 did it. Fixed in r12-5552.

Here's a new WIP patch for __is_constant_evaluated.