Why are unions which have members with differing access control not standard-layout?

Question

§9.0

7. A class S is a standard-layout class if it:

(7.3) has the same access control (Clause 11 ) for all non-static data members,

8 A standard-layout struct is a standard-layout class defined with the class-key struct or the class-key class . A standard-layout union is a standard-layout class defined with the class-key union .

AFAICT, §9.0.7.3 exists because of §9.2.13

13 Nonstatic data members of a (non-union) class with the same access control (Clause 11 ) are allocated so that later members have higher addresses within a class object. The order of allocation of non-static data members with different access control is unspecified (Clause 11 ). Implementation alignment requirements might cause two adjacent members not to be allocated immediately after each other; so might requirements for space for managing virtual functions ( 10.3 ) and virtual base classes ( 10.1 ).

However, this does not seem to apply to unions, as all (non-static data) members of a union have the same address. Is this a defect in the standard? Or is there some compelling reason why that I'm not seeing?

Answer 1

It looks to me like the benefit of allowing multiple access specifiers in the union case were not outweighed by the cost of the inconsistency and additional words introduced.