std::compare_three_way_result(3)

NAME
std::compare_three_way_result - std::compare_three_way_result

Synopsis
   Defined in header <compare>
   template< class T, class U = T > (since C++20)
   struct compare_three_way_result;
   Let t and u denote lvalue of const std::remove_reference_t<T> and const
   std::remove_reference_t<U> respectively, if the expression t <=> u is well-formed,
   provides the member typedef type equal to decltype(t <=> u), otherwise there is no
   member type.
   If the program adds specializations for std::compare_three_way_result, the behavior
   is undefined.

Member types
   Name Definition
   type the result type of operator<=> on const-qualified lvalue of T and U

Helper types
   template< class T, class U = T >
   using compare_three_way_result_t = compare_three_way_result<T, (since C++20)
   U>::type;

Possible implementation
   // recommended by Casey Carter
   // see also: https://github.com/microsoft/STL/pull/385#discussion_r357894054
   template<class T, class U = T>
   using compare_three_way_result_t = decltype(
       std::declval<const std::remove_reference_t<T>&>() <=>
       std::declval<const std::remove_reference_t<U>&>()
   );
   template<class T, class U = T>
   struct compare_three_way_result {};
   template<class T, class U>
       requires requires { typename compare_three_way_result_t<T, U>; }
   struct compare_three_way_result<T, U>
   {
       using type = compare_three_way_result_t<T, U>;
   };

Example
// Run this code
 #include <compare>
 #include <iostream>
 #include <type_traits>
 template<class Ord>
 void print_cmp_type()
 {
     if constexpr (std::is_same_v<Ord, std::strong_ordering>)
         std::cout << "strong ordering\n";
     else if constexpr (std::is_same_v<Ord, std::weak_ordering>)
         std::cout << "weak ordering\n";
     else if constexpr (std::is_same_v<Ord, std::partial_ordering>)
         std::cout << "partial ordering\n";
     else
         std::cout << "illegal comparison result type\n";
 }
 int main()
 {
     print_cmp_type<std::compare_three_way_result_t<int>>();
     print_cmp_type<std::compare_three_way_result_t<double>>();
 }

Output:
 strong ordering
 partial ordering

See also
   partial_ordering the result type of 3-way comparison that supports all 6 operators,
   (C++20) is not substitutable, and allows incomparable values
                    (class)
   weak_ordering the result type of 3-way comparison that supports all 6 operators
   (C++20) and is not substitutable
                    (class)
   strong_ordering the result type of 3-way comparison that supports all 6 operators
   (C++20) and is substitutable
                    (class)