using-declaration is not a member vs [class.member.lookup]

The lookup set for N in C, called S(N,C), consists of two component sets: the declaration set, a set of members named N; and the subobject set, a set of subobjects where declarations of these members were found (possibly via using-declarations).

Although the using-declaration declares a synonymous name for the declarations nominated by it, however, the using-declaration does not declare a member, hence the synonymous name is not the name of a member(i.e, a using-declaration that is found is not a declaration of a member).

A member-declaration does not declare new members of the class if it is

• a using-declaration ([namespace.udecl]), or

For any other member-declaration, each declared entity that is not an unnamed bit-field is a member of the class, and each such member-declaration shall either declare at least one member name of the class or declare at least one unnamed bit-field.

struct A{
    int a;
};
struct B{
    char a;
};
struct C:A,B{
    using A::a;
};
C c;
c.a = 0;

Is the set of subobjects of S(a,C) {C} or{A}? There are some contradictions here.

Start with the [class.member.lookup] wording and the cited rules for using-declarations and member-declarations. Compare those passages with the A, B, and C example in the issue. Done means the apparent contradiction is resolved by a clear interpretation or the relevant standard wording is clarified.