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[class.member.lookup] Clarify "unambiguous base class" #5173

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xmh0511 opened this issue Dec 27, 2021 · 2 comments · May be fixed by #7306
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[class.member.lookup] Clarify "unambiguous base class" #5173

xmh0511 opened this issue Dec 27, 2021 · 2 comments · May be fixed by #7306

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@xmh0511
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xmh0511 commented Dec 27, 2021 •
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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
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It seems buggy to me to say "non-dependent" here, which would indicate that a dependent base class is ambiguous.

@frederick-vs-ja
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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.

@frederick-vs-ja frederick-vs-ja linked a pull request Oct 16, 2024 that will close this issue
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[class.mi] Define ambiguous/unambiguous base class frederick-vs-ja/draft
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@xmh0511 @frederick-vs-ja