[[no_unique_address]] with duplicate type about cxx-abi HOT 2 OPEN

The current rule is consistent with the rule for base classes:

struct a { int n; };
struct e {};
struct x : e {};
struct y : e {};
struct s1 : x, y, a {}; // sizeof(s1) == 4
struct s2 : a, x, y {}; // sizeof(s2) == 8

If we used the s1 layout for s2, then converting all of s2's base classes into [[no_unique_address]] members would change its layout.

So we need to decide which we care more about: that a sequence of bases can be transformed into a sequence of [[no_unique_address]] members with no change in layout (which we can only guarantee in the absence of dynamic base classes, due to the special layout rules for virtual bases and primary base classes), or that we don't waste space on empty [[no_unique_address]] members whenever possible.

Here is perhaps a more compelling example justifying the current approach:

struct noncopyable {};
struct alloc : noncopyable {};
struct base : noncopyable { int n; };
struct data : noncopyable { int k; };
struct z : base {
  [[no_unique_address]] alloc x;
  data m;
};

It is desirable for the layout of z to match the layout of

struct z : base, alloc {
  data m;
};

... but if we consider placing alloc at offsets that would not be considered for a base class, we cannot guarantee that.

Perhaps we could consider additional offsets only for [[no_unique_address]] members that do not occur as part of a sequence of such members at the start of a class, on the basis that such cases have no corresponding base class layout to be compatible with. (And I'll open a separate issue that we should consider more offsets for empty subobjects in general.)

from cxx-abi.

Filed #77 to capture that we should consider nonzero offsets within the class when performing EBO.

from cxx-abi.