class layout for [[no_unique_address]]

[zygoloid]

C++20 adds a [[no_unique_address]] attribute, which allows EBO layout to be requested for non-static data members. We need to update the ABI document to describe how it affects class layout.

As a concrete goal, we should aim to ensure that these two classes are laid out the same:

struct A : T1, T2, ... { ... };
struct A { [[no_unique_address]] T1 t1; [[no_unique_address]] T2 t2; ... };

... in all cases where that is possible. (There are cases where it is not: for example, if A has a primary base class other than T1 and any prior base class is non-empty.)

[zygoloid]

The paper introducing this feature is here:

https://htmlpreview.github.io/?https://github.com/zygoloid/wg21papers/blob/master/done/p0840r2.html