template< class T, class U = T >
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.
type the result type of operator<=> on const-qualified lvalue of T and U [edit] Helper types
template< class T, class U = T >
using compare_three_way_result_t = compare_three_way_result<T, U>::type;
// 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>;
};[edit] Example
Output:
strong ordering partial ordering[edit] See also the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values
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