C++ use class with conversion operator as index into array

#include <cinttypes>
#include <type_traits>

template<typename Id, typename Value>
class sparse_set {

  static_assert(std::is_integral_v<Id>, "");  (1)
  static_assert(std::is_unsigned_v<Id>, "");

  Value& operator[](Id id);

  void push_back(const Value& value);
 
  // class implementation left out

};

class entity {

public:
  explicit entity(std::uint32_t id) : _id(id) {}
  ~entity() = default;

  std::uint32_t id() const {
    return _id;
  }

  operator std::uint32_t() const {  (2)
    return _id;
  }

private:
  std::uint32_t _id;

}; // class entity

int main() {
    const auto e = entity{2};

    auto set = sparse_set<entity, int>{};

    set.push_back(0);
    set.push_back(1);
    set.push_back(2);
    set.push_back(3);

    auto i = set[e];  (3)
    
    return 0;
}

I am trying to use a class with a conversion operator to std::uint32_t (2) as an index into a container class (3). Accessing an element with an instance of that class works and i get the right element.

But testing the class with a static_assert and std::is_unsigned_v and std::is_integral_v results in an assertion failure.

I need assertions to make sure Id can be used as an index.

When I static_assert with std::uint32_t everything works so I would expect the conversion operator to work aswell.

std::is_unsigned and std::is_integral only works for primitive types, it doesn't work for classes, even if they are implicitly convertible to a type they support. You can solve this by two ways:

1. Create your own trait:

   #include <type_traits>
   
   // ...
   
   using uint16 = std::uint16_t;
   using uint32 = std::uint32_t;
   
   class entity { /* ... */ };
   
   template <typename T>
   struct is_valid_index : std::is_unsigned<T> {};
   template <>
   struct is_valid_index<entity> : std::true_type {};
   
   template <typename T>
   constexpr auto is_valid_index_v = is_valid_index<T>::value;
   
   template<typename Id, typename Value>
   class sparse_set {
       static_assert(is_valid_index_v<Id>, "");
       // ...
   };
   
   // ...
   

2. Remove std::is_unsigned completely and use std::is_convertible instead:

   #include <type_traits>
   
   // ...
   
   using uint16 = std::uint16_t;
   using uint32 = std::uint32_t;
   
   class entity { /* ... */ };
   
   template<typename Id, typename Value>
   class sparse_set {
       static_assert(std::is_convertible_v<Id, uint32>, "");
       // ...
   };
   
   // ...