Layout for [[no_unique_address]] members is not optimal under Cfront-like layout

[cpplearner]

Issue Summary

When a struct contains multiple [[no_unique_address]] members of the same type, these members occupy storage, making the struct larger than needed, under Cfront-like layout.

It appears that the byte offsets of empty members are computed before those of non-empty members, and thus the initial offset of the latter depends on the final offset of the former. That is, an empty member cannot share storage with a non-empty member other than the first byte of the first non-empty member.

Clang, GCC, and MSVC (with [[msvc::no_unique_address]]) are able to produce the optimal layout for the repro below, as does EDG under Itanium ABI.

This causes discrepancy between Visual Studio's EDG-based IntelliSense and MSVC compiler.

Reproducing Source Code

struct Empty {};

struct S1 {
    [[no_unique_address]] Empty mem1;
    [[no_unique_address]] Empty mem2;
    int mem_int;
};

static_assert(sizeof(S1) == sizeof(int));

Command-line Options

cpfe --ms_c++20 test-no-unique-address.cpp

Type of Issue

[Front End] This code SHOULD compile with these options.

Additional Details

No response

Reproducing (Standard) Configuration(s)

windows-msvc-debug

Reproducing (Non-standard) Configuration

No response

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