When the STL became part of the standard library (
http://www.stlport.org/resources/StepanovUSA.html ), there was a question of how to handle algorithms that sort containers, or that perform a binary search, or in some other way, need to know whether “a” is less-than, equal-to, or greater-than “b”. The algorithms have to work on primitive types, and user defined types as efficiently as possible. They eventually decided to only require a function for “less-than.” And if “a < b” returns false, and “b < a” also returns false, then “a” and “b” are considered equal.
There are times that doesn’t work, so documentation usually has a footnote that (1) certain algorithms require a partial ordering and not necessarily a total ordering, and (2) to use those algorithms, you must implement less-than, but any other comparison operator is ignored by the algorithm; instead, less-than is used to figure out greater-than and equal-to as needed. This was considered better than requiring programmers to implement a collection of comparison operators, and trusting those programmers to make those operators act consistently with each other (e.g., never say that “a” and “b” are both less-than and greater-than each other).
The spaceship operator seems to address this specific case ( https://open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0515r0... ). According to Herb Sutter (note that his name is on the proposal), “We added the C++20 spaceship operator to the language, but we also applied it throughout the C++ standard library and that made the library specification nearly 20 pages shorter — a net reduction” ( https://herbsutter.com/2020/12/ ).