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namespace unconstexpr
{
   class uniq_value
   {
       struct Detail
       {
           template <unsigned N>
           struct Flag
           {
               friend constexpr bool adl_flag(Flag);
           };
       
           template <unsigned N>
           struct Writer
           {
               friend constexpr bool adl_flag(Flag)
               {
                   return true;
               }

               static constexpr unsigned value = N;
           };

           template <unsigned N = 0, bool = adl_flag(Flag{})>
           static constexpr unsigned reader(int, unsigned r = reader<N+1>(0))
           {
               return r;
           }

           template <unsigned N = 0>
           static constexpr unsigned reader(float)
           {
               return Writer::value;
           }
       };

   public:
       template <unsigned N = Detail::reader(0)>
       static constexpr unsigned value = N;
   };
}

namespace unconstexpr
{
   namespace detail…

a c++ header library that implements variant constexpr functions and types. - DaemonSnake/unconstexpr

A detailed post that shows how constant-expressions in C++ are not as constant as they might appear.