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), decltype(t < u) and decltype(u < t) 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.

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. See CustomizationPointObject for details.

Example

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also