[class.member.lookup] Clarify "unambiguous base class"

[xmh0511]

We refer to "unambiguous base class" in this document many times but we never clearly define the concept in this document. I think we can define this concept with the help of [class.member.lookup] p5.3.

[class.member.lookup] p5.3 says

Otherwise, the new S(N, C) is a lookup set with the shared set of declarations and the union of the subobject sets.

We can say

A non-dependent class type B is an unambiguous base class of class C, if and only if C is derived from B and the declaration set and the subobject set of S(B, C) consist of a unique declaration of B and a unique member subobject, respectively.

[frederick-vs-ja]

It seems buggy to me to say "non-dependent" here, which would indicate that a dependent base class is ambiguous.

[frederick-vs-ja]

Maybe we can add something to [class.derived.general]/3:

If class B is a base class of class D, B is an unambiguous base class of D if there is only one (possibly recursive) base class subobject of type B of an object of type D, otherwise, B is an ambiguous base class of D.