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
   compare_partial_order_fallback(T&& t, U&& u) noexcept(/* see below
   */);
   Performs three-way comparison on t and u and produces a result of type
   std::partial_ordering, even if the operator <=> is unavailable.
   Let t and u be expressions and T and U denote decltype((t)) and decltype((u))
   respectively, std::compare_partial_order_fallback(t, u) is expression-equivalent to:
     * If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
          * 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 t == u, t < u, and u < t are all well-formed and convertible
                        to bool, 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.
  Expression-equivalent
   Expression e is expression-equivalent to expression f, if
     * e and f have the same effects, and
     * either both are constant subexpressions or else neither is a constant
       subexpression, and
     * either both are potentially-throwing or else neither is potentially-throwing
       (i.e. noexcept(e) == noexcept(f)).
  Customization point objects
   The name std::compare_partial_order_fallback denotes a customization point object,
   which is a const function object of a literal semiregular class type. For exposition
   purposes, the cv-unqualified version of its type is denoted as
   __compare_partial_order_fallback_fn.
   All instances of __compare_partial_order_fallback_fn are equal. The effects of
   invoking different instances of type __compare_partial_order_fallback_fn on the same
   arguments are equivalent, regardless of whether the expression denoting the instance
   is an lvalue or rvalue, and is const-qualified or not (however, a volatile-qualified
   instance is not required to be invocable). Thus, std::compare_partial_order_fallback
   can be copied freely and its copies can be used interchangeably.
   Given a set of types Args..., if std::declval<Args>()... meet the requirements for
   arguments to std::compare_partial_order_fallback above,
   __compare_partial_order_fallback_fn models
     * std::invocable<__compare_partial_order_fallback_fn, Args...>,
     * std::invocable<const __compare_partial_order_fallback_fn, Args...>,
     * std::invocable<__compare_partial_order_fallback_fn&, Args...>, and
     * std::invocable<const __compare_partial_order_fallback_fn&, Args...>.
   Otherwise, no function call operator of __compare_partial_order_fallback_fn
   participates in overload resolution.

Example
    This section is incomplete
    Reason: no example

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