If the class definition
does not explicitly declare an == operator function,
but declares a defaulted three-way comparison operator function,
an == operator function is declared implicitly
with the same access as the three-way comparison operator function.
The implicitly-declared == operator for a class X
is an inline member and is defined as defaulted in the definition of X.
If the three-way comparison operator function
is declared as a non-static const member,
the implicitly-declared == operator function is a member of the form
bool X::operator==(const X&)const;
Otherwise, the implicitly-declared == operator function is of the form
C is a class type with an == operator
defined as defaulted in the definition of C,
x == x is well-formed when contextually converted to bool,
all of C's base class subobjects and non-static data members
have strong structural equality, and
C has no mutable or volatile subobjects.
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 xi be an lvalue denoting the ith element
in the expanded list of subobjects for an object x
(of length n),
where xi 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
appear more than once in the expanded list of subobjects.