https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78420
--- Comment #23 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Jason Merrill from comment #22)
> Hmm, this seems to work:
>
> typedef decltype(sizeof(1)) size_t;
>
> constexpr bool less (const int*a, const int*b)
> {
> if (__builtin_constant_p (a < b))
> return a < b;
> return (size_t)a < (size_t)b;
> }
>
> int ar[3];
> int i;
>
> constexpr bool l1 = less(&ar[0], &ar[1]); // OK
> constexpr bool l2 = less(&ar[0], &i); // error, non-constant
I'd fear about jump-threading vs. __builtin_constant_p issues like PR72785 and
others, were the optimizers break the __builtin_constant_p argument and the
guarded expression appart and something different might be propagated into them
or evaluated differently etc.
One possibility would be a __builtin_constant_p-like builtin that would fold to
false much earlier than the current __builtin_constant_p, e.g. during
gimplification.
Or use p0595r0 if (constexpr () && a < b) return a < b; or similar builtin
if (__builtin_in_constexpr_p () && a < b) return a < b;
Again, fold this builtin to true if inside constexpr.c, and to false say in the
C++ gimplification langhook.
