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Showing content from https://en.cppreference.com/w/cpp/language/../utility/compare/compare_partial_order_fallback.html below:

std::compare_partial_order_fallback - cppreference.com

inline namespace /* unspecified */ { inline constexpr /* unspecified */ compare_partial_order_fallback = /* unspecified */; } (since C++20)

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 */); (since C++20)

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 : â¯â¯â¯â¯â¯â¯â¯â¯â¯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.

[edit] Example [edit] 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
(P2167R3) C++20 the fallback mechanism only required
return types to be convertible to bool constraints strengthened LWG 3465 C++20 the fallback mechanism did not require u < t to be well-formed required LWG 4157 C++20 P2167R3 overwrote the resolution of LWG issue 3465 restored the resolution [edit] See also performs 3-way comparison and produces a result of type std::partial_ordering
(customization point object)[edit]

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