On Wed, 19 Feb 2020 at 15:42, Klaus Doldinger
<klaus.doldinger64@xxxxxxxxx> wrote:
>
>
>
> Am 19.02.20 um 13:43 schrieb Jonathan Wakely:
> > On Wed, 19 Feb 2020 at 11:17, Marc Glisse wrote:
> >>
> >> On Wed, 19 Feb 2020, Klaus Doldinger wrote:
> >>
> >>> Am 19.02.20 um 11:02 schrieb Jonathan Wakely:
> >>>> On Wed, 19 Feb 2020 at 07:53, Klaus Doldinger
> >>>> <klaus.doldinger64@xxxxxxxxx> wrote:
> >>>>>
> >>>>> The following was possible in v9:
> >>>>>
> >>>>> template<auto N>
> >>>>> struct A {
> >>>>> constexpr auto size() const {
> >>>>> return N;
> >>>>> }
> >>>>> };
> >>>>>
> >>>>> template<typename A>
> >>>>> void foo(const A& a) {
> >>>>> constexpr auto s = a.size();
> >>>>> }
> >>>>>
> >>>>> int main() {
> >>>>> A<10> x1;
> >>>>> foo(x1);
> >>>>> }
> >>>>>
> >>>>> In v10 this is rejected.
> >>>>>
> >>>>> Looks like a bug?
> >>
> >> Clang also rejects it.
> >>
> >>>> No, I don't think so. The parameter a is not a constant expression, so
> >>>> a.size() is not a constant expression, so can't be used to initialize
> >>>> a constexpr variable.
> >>
> >> Why not make size() static?
> >>
> >>> That would be weird.
> >>> If so, the follwing would be impossible:
> >>>
> >>> std::array<int, 10> a;
> >>> std::array<int, a.size()> b;
> >>
> >> You can still use decltype and tuple_size to access that information.
> >
> > And it works if 'a' is usable in a constant expression, e.g. because
> > it has static storage duration.
> >
> > It doesn't work if it's an automatic variable, or if you pass it by
> > reference to a non-constexpr function, which is the problem in the
> > OP's original example.
>
>
> Well, the following works:
>
> A<10> x1;
> A<x1.size()> x2;
> std::array<int, 10> a;
> std::array<int, a.size()> b;
>
> Here x1 is not a constant expression, and it is accepted. Why?
>
> Please explain what the difference is to what in function foo() happens.
No. Please take such questions to somewhere more suitable, like stackoverflow.
> If function foo() is changed to
>
> template<typename T>
> void foo(T a) {
> constexpr auto s = a.size();
> }
>
> it is also expected.
>
> Thats not consistent I would say.
>
>
