11 Classes [class]

11.11 Comparisons [class.compare]

11.11.4 Secondary comparison operators [class.compare.secondary]

A secondary comparison operator is a relational operator ([expr.rel]) or the != operator.
A defaulted operator function ([over.binary]) for a secondary comparison operator @ shall have a declared return type bool.
The operator function with parameters x and y is defined as deleted if
  • overload resolution ([over.match]), as applied to x @ y, does not result in a usable candidate, or
  • the candidate selected by overload resolution is not a rewritten candidate.
Otherwise, the operator function yields x @ y.
The defaulted operator function is not considered as a candidate in the overload resolution for the @ operator.
[Example:
struct HasNoLessThan { };

struct C {
  friend HasNoLessThan operator<=>(const C&, const C&);
  bool operator<(const C&) const = default;             // OK, function is deleted
};
— end example]