arity.h

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#pragma once
#include <cstddef>
#include <type_traits>
 
namespace librepr::detail {
struct Universal {
  template <typename T>
  /* implicit */ constexpr operator T();  // NOLINT
};
 
template <typename T>
  requires std::is_aggregate_v<T>
struct ArityImpl {
  template <typename... Fillers>
  static consteval auto array_length(auto... parameters) {
    if constexpr (requires { T{parameters..., {Fillers{}..., Universal{}}}; }) {
      return array_length<Fillers..., Universal>(parameters...);
    } else {
      return sizeof...(Fillers);
    }
  }
 
  static consteval auto arity_simple(auto... parameters) {
    if constexpr (requires { T{parameters..., Universal{}}; }) {
      return arity_simple(parameters..., Universal{});
    } else {
      return sizeof...(parameters);
    }
  }
 
  template <typename... Trails>
  static consteval auto arity_simple_ag(auto... parameters) {
    if constexpr (requires { T{parameters..., {Universal{}, Universal{}}, Trails{}..., Universal{}}; }) {
      return arity_simple_ag<Trails..., Universal>(parameters...);
    } else {
      return sizeof...(parameters) + sizeof...(Trails) + 1;
    }
  }
 
  static consteval auto arity(std::size_t minus = 0, auto... parameters) {
    if constexpr (requires { T{parameters..., {Universal{}, Universal{}}}; }) {
      if constexpr (arity_simple_ag(parameters...) != arity_simple(parameters...)) {
        minus += array_length(parameters...) - 1;
      }
 
      return arity(minus, parameters..., Universal{});
    } else if constexpr (requires { T{parameters..., Universal{}}; }) {
      return arity(minus, parameters..., Universal{});
    } else {
      return sizeof...(parameters) - minus;
    }
  }
};
 
template <typename T>
  requires std::is_aggregate_v<T>
inline constexpr std::size_t arity = ArityImpl<T>::arity();
 
}  // namespace librepr::detail