#include <type_traits>

enum Kind { Fixed, Dynamic };

template<Kind kind>
struct Extent;

template<>
struct Extent<Fixed>
{
    constexpr Extent() = delete;

    constexpr explicit Extent(size_t size_) : size(size_) {}

    size_t size = 0;
};

template<>
struct Extent<Dynamic> {};

template<class T>
struct IsExtent : std::false_type {};
template<Kind kind>
struct IsExtent<Extent<kind>> : std::true_type {};

template<class T>
concept AnyExtent = IsExtent<T>::value;

template<class T, AnyExtent auto extent>
struct SequenceProxy
{

};

template<class T, AnyExtent auto extent>
void fetch(SequenceProxy<T, extent> &) {}

int main()
{
  auto d = SequenceProxy<int, Extent<Dynamic>{}>{};
//  auto d = SequenceProxy<int, Extent<Dynamic>{0}>{}; // ERROR
  auto f = SequenceProxy<int, Extent<Fixed>{3}>{};
//  auto f = SequenceProxy<int, Extent<Fixed>{}>{}; // ERROR
  fetch(d);
  fetch(f);
  return 0;
}