If both operands have arithmetic types,
or one operand has integral type and
the other operand has unscoped enumeration type,
the usual arithmetic conversions are applied to the operands.
Otherwise, if the operands have integral type,
the result is of type std::strong_ordering. The result is
std::strong_ordering::equal
if both operands are arithmetically equal,
std::strong_ordering::less
if the first operand is arithmetically
less than the second operand,
and
std::strong_ordering::greater
otherwise.
Otherwise, the operands have floating-point type, and
the result is of type std::partial_ordering. The expression a <=> b yields
std::partial_ordering::less
if a is less than b,
std::partial_ordering::greater
if a is greater than b,
std::partial_ordering::equivalent
if a is equivalent to b,
and
std::partial_ordering::unordered otherwise.
If both operands have the same enumeration type E,
the operator yields the result of
converting the operands to the underlying type of E
and applying <=> to the converted operands.
If at least one of the operands is of pointer type and
the other operand is of pointer or array type,
array-to-pointer conversions ([conv.array]),
pointer conversions ([conv.ptr]),
and
qualification conversions
are performed on both operands
to bring them to their composite pointer type ([expr.type]).
After the conversions, the operands shall have the same type.
If the composite pointer type is an object pointer type,
p <=> q is of type std::strong_ordering.
If two pointer operands p and q compare equal ([expr.eq]),
p <=> q yields std::strong_ordering::equal;
if p and q compare unequal,
p <=> q yields
std::strong_ordering::less
if q compares greater than p
and
std::strong_ordering::greater
if p compares greater than q ([expr.rel]).
The three comparison category types ([cmp.categories])
(the types
std::strong_ordering,
std::weak_ordering, and
std::partial_ordering)
are not predefined;
if the header <compare> ([compare.syn])
is not imported or included prior to a use of such a class type –
even an implicit use in which the type is not named
(e.g., via the auto specifier
in a defaulted three-way comparison
or use of the built-in operator) – the program is ill-formed.