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From: "hstong at ca dot ibm.com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/96105] New: GCC not consistent on whether no_unique_address
 array is an empty data member
Date: Tue, 07 Jul 2020 22:30:34 +0000
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X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: accepts-invalid, rejects-valid, wrong-code
X-Bugzilla-Severity: normal
X-Bugzilla-Who: hstong at ca dot ibm.com
X-Bugzilla-Status: UNCONFIRMED
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96105

            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 ar=
ray
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=3Dc++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=3D/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=3D/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=3D/opt/wandbox/gcc-head
--enable-languages=3Dc,c++ --disable-multilib --without-ppl --without-cloog=
-ppl
--enable-checking=3Drelease --disable-nls --enable-lto
LDFLAGS=3D-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)=