Constexpr counter that works on GCC 8, and is not restricted to namespace scope

Question

I\'m trying to learn some arcane stateful template metaprogramming tricks.
_{(Here\'s why I want to learn it. Unfortunately this library doesn\'t work on GCC}

Answer 1

As I didn't find fully working version, I improved @anthony-williams solution by adding custom start and step value, and also added some compatibility with MVSC, clang and gcc (thanx to @segfault comment).
I hope it may me helpful.
code tests

namespace detail
{

template <typename T> class Flag
{
    struct Dummy
    {
        constexpr Dummy()
        {
        }
        friend constexpr void adl_flag(Dummy);
    };

    template <bool>
    struct Writer
    {
        friend constexpr void adl_flag(Dummy)
        {
        }
    };

    template <class Dummy, int = (adl_flag(Dummy{}), 0) >
    static constexpr bool Check(int)
    {
        return true;
    }

    template <class Dummy>
    static constexpr bool Check(short)
    {
        return false;
    }

public:

    template <class Dummy = Dummy, bool Value = Check<Dummy>(0)>
    static constexpr bool ReadSet()
    {
        Writer<Value && 0> tmp{};
        (void)tmp;
        return Value;
    }

    template <class Dummy = Dummy, bool Value = Check<Dummy>(0)>
    static constexpr int Read()
    {
        return Value;
    }
};

template <typename T, int I>
struct Tag
{

    constexpr int value() const noexcept
    {
        return I;
    }
};

template<typename T, int N, int Step, bool B>
struct Checker
{
    static constexpr int currentval() noexcept
    {
        return N;
    }
};

template<typename T, int N, int Step>
struct CheckerWrapper
{
    template<bool B = Flag<Tag<T, N>>{}.Read(), int M = Checker<T, N, Step, B>{}.currentval() >
    static constexpr int currentval()
    {
        return M;
    }
};

template<typename T, int N, int Step>
struct Checker<T, N, Step, true>
{
    template<int M = CheckerWrapper<T, N + Step, Step>{}.currentval() >
    static constexpr int currentval() noexcept
    {
        return M;
    }
};

template<typename T, int N, bool B = Flag<Tag<T, N>>{}.ReadSet() >
struct Next
{
    static constexpr int value() noexcept
    {
        return N;
    }
};

}

template <class Tag = void, int Start = 0, int Step = 1>
class constexpr_counter
{
public:
    template <int N = detail::CheckerWrapper<Tag, Start, Step>{}.currentval()>
    static constexpr int next()
    {
        return detail::Next<Tag, N>{}.value();
    }
};

examples:

using counter_A_0_1 = constexpr_counter<struct TagA, 0, 1>;
constexpr int a0 = counter_A_0_1::next();
constexpr int a1 = counter_A_0_1::next();
constexpr int a2 = counter_A_0_1::next();
static_assert(a0 == 0);
static_assert(a1 == 1);
static_assert(a2 == 2);

using counter_B_0_1 = constexpr_counter<struct TagB, 0, 1>;
constexpr int b0 = counter_B_0_1::next();
constexpr int b1 = counter_B_0_1::next();
constexpr int b2 = counter_B_0_1::next();
static_assert(b0 == 0);
static_assert(b1 == 1);
static_assert(b2 == 2);

using counter_C_2_1 = constexpr_counter<struct TagC, 2, 1>;
constexpr int c0 = counter_C_2_1::next();
constexpr int c1 = counter_C_2_1::next();
constexpr int c2 = counter_C_2_1::next();
static_assert(c0 == 2);
static_assert(c1 == 3);
static_assert(c2 == 4);

using counter_D_4_1 = constexpr_counter<struct TagD, 4, 1>;
constexpr int d0 = counter_D_4_1::next();
constexpr int d1 = counter_D_4_1::next();
constexpr int d2 = counter_D_4_1::next();
static_assert(d0 == 4);
static_assert(d1 == 5);
static_assert(d2 == 6);

using counter_E_5_3 = constexpr_counter<struct TagE, 5, 3>;
constexpr int e0 = counter_E_5_3::next();
constexpr int e1 = counter_E_5_3::next();
constexpr int e2 = counter_E_5_3::next();
static_assert(e0 == 5);
static_assert(e1 == 8);
static_assert(e2 == 11);

using counter_F_2_m3 = constexpr_counter<struct TagF, 2, -3>;
constexpr int f0 = counter_F_2_m3::next();
constexpr int f1 = counter_F_2_m3::next();
constexpr int f2 = counter_F_2_m3::next();
static_assert(f0 == 2);
static_assert(f1 == -1);
static_assert(f2 == -4);

Answer 2

The body of a constexpr function template must yield the same answer for all instatiations with the same template parameters and same arguments. You need to add a level of indirection, so the calculation can happen in the default argument of a template parameter dependent on the first.

See https://gcc.godbolt.org/z/GHfKKf

namespace Meta
{
    template <typename T,int I> struct Tag {};

    template<typename T,int N,bool B>
    struct Checker{
        static constexpr int currentval() noexcept{
            return N;
        }
    };

    template<typename T,int N>
    struct CheckerWrapper{
        template<bool B=Flag<Tag<T,N>>::Read(),int M=Checker<T,N,B>::currentval()>
        static constexpr int currentval(){
            return M;
        }
    };

    template<typename T,int N>
    struct Checker<T,N,true>{
        template<int M=CheckerWrapper<T,N+1>::currentval()>
        static constexpr int currentval() noexcept{
            return M;
        }
    };

    template<typename T,int N,bool B=Flag<Tag<T,N>>::ReadSet()>
    struct Next{
        static constexpr int value() noexcept{
            return N;
        }
    };

    template <typename T> class TaggedCounter
    {
      public:
        template <int N=CheckerWrapper<T,0>::currentval()> static constexpr int Value(){
            return Next<T,N>::value();
        }
    };
}