Initially use [[msvc::no_unique_address]] for some C++23 components

[frederick-vs-ja]

Towards #1364.

Applied:

• in_value_result
• out_value_result
• mdspan
• join_with_view's sentinel
• enumerate_view's iterator and sentinel
• chunk_view's iterator
• slide_view's sentinel
• elements_of

Not yet applied due to DevCom-10588147 and possibly related issues.

• views, as they have view_interface as their empty bases.
• some iterators with empty bases due to conditionally existing iterator_category.
• chunk_by_view's iterator, possibly because the non-potentially-overlapping member is listed first.

Not yet applied because it's unclear to me how to reuse padding bytes in tuple

• zip-family

Not yet applied due to another unclear reasons

• expected

Other changes:

• remove /* [[no_unique_address]] */ for _Index_type of repeat_view's iterator because the _Index_type never has padding bytes
• move the test coverage added in <mdspan>: More EBO #3847 to a separated test file
• attempt to test NVCC for these changes failed and reverted.

[StephanTLavavej]

This is blocked by DevCom-10754777 VSO-2266748 "Wrong code generation when initializing a variable of class type that contains multiple instances of the same empty class in its layout", fixed by MSVC-PR-583942 on 2024-10-10, shipping in 17.13p2.

Click to expand fully reduced repro:

C:\Temp>type meow.cpp

#include <cassert>

using Extents = signed char;

struct Holder {
    constexpr Holder(const Extents& e) : exts(e) {}
    [[msvc::no_unique_address]] Extents exts;
};

struct Empty {};

struct Mdspan {
    [[msvc::no_unique_address]] Holder holder;
    [[msvc::no_unique_address]] Empty acc{};
    [[msvc::no_unique_address]] int val{1729};
};

int main() {
    Extents extents{7};
    Mdspan md{extents};
    assert(md.holder.exts == extents);
}

17.12 Preview 3 x64:

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /d1reportSingleClassLayoutMdspan meow.cpp && meow
meow.cpp

class Mdspan    size(4):
        +---
 0      | Holder holder
 0      | Empty acc
 0      | val
        +---
Assertion failed: md.holder.exts == extents, file meow.cpp, line 21

Current prod/fe x64:

C:\Temp>cl /EHsc /nologo /W4 /std:c++latest /MTd /Od /d1reportSingleClassLayoutMdspan meow.cpp && meow
meow.cpp

class Mdspan    size(8):
        +---
 0      | Holder holder
 0      | Empty acc
        | <alignment member> (size=3)
 4      | val
        +---

C:\Temp>

We talked about this in Discord, and I now agree with @AlexGuteniev that all use of [[msvc::no_unique_address]] should be avoided until we have this fix - even though only mdspan is directly affected now, user usage could reveal other problems, and since it happens only at runtime in a catastrophic way, this isn't worth the risk when the compiler is known to be unreliable.

• Merge with the VS 2022 17.13 Preview 2 toolset update.
• Verify that llvm-project\libcxx\test\std\containers\views\mdspan\mdspan passes again.

[StephanTLavavej]

Resolved a trivial merge conflict in <xmemory> with #4987. (Set to draft mode to avoid running the CI while this PR is blocked.)