std::partial_order
From cppreference.com
Defined in header <compare>


template< class T > constexpr std::partial_ordering partial_order(const T& a, const T& b); 
(since C++20)  
Compares two values using 3way comparison and produces a result of type std::partial_ordering
Specifically,
 If the expression a <=> b is wellformed and its result is convertible to std::partial_ordering, returns that result.
 Otherwise, if the expression a <=> b is wellformed, but its result is not convertible to std::partial_ordering, then the function is defined as deleted.
 Otherwise, if the expression a <=> b is illformed, but the expressions a == b and a < b are both wellformed and convertible to bool,
 if a == b is true, returns std::partial_ordering::equivalent
 otherwise, if a < b is true, returns std::partial_ordering::less
 otherwise, returns std::partial_ordering::greater
 Otherwise, the function is defined as deleted.
Parameters
a, b    values to compare 
Return value
A value of type std::partial_ordering, as described above.
Notes
Example
This section is incomplete Reason: no example 
See also
(C++20) 
the result type of 3way comparison that supports all 6 operators, is not substitutable, and allows incomparable values (class) 
(C++20) 
performs 3way comparison and produces a result of type std::strong_ordering (function template) 
(C++20) 
performs 3way comparison and produces a result of type std::weak_ordering (function template) 
(C++20) 
performs 3way comparison and produces a result of type std::weak_equality (function template) 