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

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  struct Tag {};

    template
    struct Checker{
        static constexpr int currentval() noexcept{
            return N;
        }
    };

    template
    struct CheckerWrapper{
        template>::Read(),int M=Checker::currentval()>
        static constexpr int currentval(){
            return M;
        }
    };

    template
    struct Checker{
        template::currentval()>
        static constexpr int currentval() noexcept{
            return M;
        }
    };

    template>::ReadSet()>
    struct Next{
        static constexpr int value() noexcept{
            return N;
        }
    };

    template  class TaggedCounter
    {
      public:
        template ::currentval()> static constexpr int Value(){
            return Next::value();
        }
    };
}