A defaulted relational ([expr.rel]) or equality ([expr.eq]) operator function
for some 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 (also considering synthesized candidates with reversed order of parameters ([over.match.oper])), results in an ambiguity or a function that is deleted or inaccessible from the operator function, or

Otherwise, the operator function yields
x <=> y @ 0
if an operator<=>
with the original order of parameters was selected, or
0 @ y <=> x
otherwise.