# std::compare_partial_order_fallback

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Defined in header `<compare>` |  |
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 :
  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.

### 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

### 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

### See also

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

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