std::compare_partial_order_fallback(3)

NAME
std::compare_partial_order_fallback - std::compare_partial_order_fallback

Synopsis
   Defined in header <compare>
   inline namespace /* unspecified */ {
       inline constexpr /* unspecified */ (since C++20)
           compare_partial_order_fallback = /* unspecified */;
   }
   Call signature
   template< class T, class U >
       requires /* see below */
   constexpr std::partial_ordering (since C++20)
       compare_partial_order_fallback( T&& t, U&& u ) noexcept(/* see
   below */);
   Performs three-way comparison on subexpressions t and u and produces a result of
   type std::partial_ordering, even if the operator <=> is unavailable.
   If std::decay_t<T> and std::decay_t<U>> are the same type,
   std::compare_partial_order_fallback(t, u) is expression-equivalent to:
     * std::partial_order(t, u), if it is a well-formed expression;
     * otherwise, t == u ? std::partial_ordering::equivalent :
       t < u ? std::partial_ordering::less :
       u < t ? std::partial_ordering::greater :
       std::partial_ordering::unordered
   if the expressions t == u, t < u and u < t are all well-formed and each of
   decltype(t == u) and decltype(t < u) models boolean-testable, except that t and u
   are evaluated only once.
   In all other cases, std::compare_partial_order_fallback(t, u) is ill-formed, which
   can result in substitution failure when it appears in the immediate context of a
   template instantiation.

Example
    This section is incomplete
    Reason: no example
   Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 2114 C++20 the fallback mechanism only required constraints strengthened
   (P2167R3) return types to be convertible to bool
   LWG 3465 C++20 the fallback mechanism did not require required
                        u < t to be well-formed

See also
   partial_order performs 3-way comparison and produces a result of type
   (C++20) std::partial_ordering
                 (customization point object)

Category:
     * Todo no example