[Bug c++/96052] New: Unlike Clang, alignment specifier is ignored on empty no_unique_address members

[hstong at ca dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96052

            Bug ID: 96052
           Summary: Unlike Clang, alignment specifier is ignored on empty
                    no_unique_address members
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, rejects-valid, wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hstong at ca dot ibm.com
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

The following fully standard (if not portable) C++ source fails to compile with
GCC and does compile with Clang when targeting the same platform. This is
indicative of an ABI incompatibility that is presumably unintended.

Compiler Explorer link: https://godbolt.org/z/RG_QKE

### SOURCE (<stdin>):
struct Q {
  struct {
  } emp alignas(8) [[no_unique_address]];
  char x;
};
struct QQ {
  char x;
  Q q;
};

struct Z {
  char x alignas(8) [[no_unique_address]];
};
struct ZZ {
  char x;
  Z z;
};

extern char qx[sizeof(QQ)];
extern char qx[16];
extern char qz[sizeof(ZZ)];
extern char qz[16];


### COMPILER INVOCATION:
g++ -fsyntax-only -std=c++20 -Wall -Wextra -pedantic-errors -xc++ -


### ACTUAL OUTPUT:
<stdin>:20:13: error: conflicting declaration 'char qx [16]'
<stdin>:19:13: note: previous declaration as 'char qx [2]'


### EXPECTED OUTPUT:
(clean compile)


### COMPILER VERSION INFO (g++ -v):
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/11.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.0 20200702 (experimental) (GCC)