https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112288
--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
// PR c++/112288
namespace {
template <typename Context, int Start = 0, int Step = 1>
class counter
{
public:
template <typename Unique>
static constexpr int next()
{
return next<Unique>(0)*Step+Start;
}
private:
template <int I>
struct slot
{
template <typename Dummy>
friend constexpr auto slot_allocated(Dummy, slot<I>);
};
template <int I>
struct allocate_slot {
template <typename Dummy>
friend constexpr auto slot_allocated(Dummy, slot<I>) {
return true;
}
enum { value = I };
};
template <typename Unique, int I = 0, int Dummy = 0, bool =
slot_allocated(Dummy, slot<I>())>
static constexpr int next(int)
{
return next<Unique, I+1>(0);
}
template <typename Unique, int I = 0>
static constexpr int next(double)
{
return allocate_slot<I>::value;
}
};
}
template <int I>
struct U;
struct C: counter<U<1>>{};
int a = C::next<struct unique_1>();
int b = C::next<struct unique_2>();
