[Bug c++/96105] New: GCC not consistent on whether no_unique_address array is an empty data member

["hstong at ca dot ibm.com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96105

            Bug ID: 96105
           Summary: GCC not consistent on whether no_unique_address array
                    is an empty data member
           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: ---

GCC treats an array marked `no_unique_address` and whose base element type is
an empty class as not an empty data member in `C` but does treat such an array
as an empty data member in `A` for the purposes of `B`. It appears that GCC's
handling of `B` is erroneous.

Compiler Explorer link: https://godbolt.org/z/XX5W3v

### SOURCE (<stdin>):
struct Empty {};

struct A {
  Empty emp [[no_unique_address]][3];
};

struct B : A {
  float f;
};

struct C {
  Empty emp [[no_unique_address]][3];
  float f;
};

extern char szc[sizeof(C)];
extern char szc[sizeof(float) * 2];  // GCC likes this
extern char szb[sizeof(B)];
extern char szb[sizeof(float) * 2];  // GCC does not like this


### COMPILER INVOCATION:
g++ -fsyntax-only -std=c++2a -Wall -Wextra -pedantic-errors -xc++ -


### ACTUAL OUTPUT:
<stdin>:19:13: error: conflicting declaration 'char szb [8]'
<stdin>:18:13: note: previous declaration as 'char szb [4]'


### 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 20200706 (experimental) (GCC)