The direct base class subobjects of C,
in the order of their declaration in the *base-specifier-list* of C,
followed by the non-static data members of C,
in the order of their declaration in the *member-specification* of C,
form a list of subobjects.

In that list, any subobject of array type is recursively expanded
to the sequence of its elements, in the order of increasing subscript.

Let be an lvalue denoting the element
in the expanded list of subobjects for an object x
(of length n),
where is
formed by a sequence of
derived-to-base conversions ([over.best.ics]),
class member access expressions ([expr.ref]), and
array subscript expressions ([expr.sub]) applied to x.

It is unspecified
whether virtual base class subobjects are compared more than once.

If the declared return type
of a defaulted three-way comparison operator function
is auto,
then the return type is deduced as
the common comparison type (see below) of
, , …, .

[ Note

: *end note*

]Otherwise,
the program will be ill-formed
if the expression <=>
is not implicitly convertible to the declared return type for any i.

—
If the return type is deduced as void,
the operator function is defined as deleted.

The return value V of type R
of the defaulted three-way comparison operator function
with parameters x and y of the same type
is determined by comparing corresponding elements
and
in the expanded lists of subobjects for x and y
until the first index i
where <=>
yields a result value where ,
contextually converted to bool, yields true;
V is converted to R.

- Otherwise, if at least one is std::weak_equality, or at least one is std::strong_equality and at least one is std::partial_ordering or std::weak_ordering, U is std::weak_equality ([cmp.weakeq]).
- Otherwise, if at least one is std::partial_ordering, U is std::partial_ordering ([cmp.partialord]).