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

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();
        }
    };
}