A defaulted relational operator function ([over.binary])
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, does not result in a usable function, or
- the operator @ cannot be applied to the return type of x <=> y.

Otherwise, the operator function yields
x <=> y @ 0.