[Bug c++/81976] New: bad is_standard_layout/has_unique_object_representations results with a chain of empty bases

["oremanj at mit dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81976

            Bug ID: 81976
           Summary: bad
                    is_standard_layout/has_unique_object_representations
                    results with a chain of empty bases
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: oremanj at mit dot edu
  Target Milestone: ---

The C++ program below defines structures that each contain a 1-byte char field
and inherit linearly from various numbers of empty bases (one empty base for X,
two for Y, three for Z). By my reading of the standard, all of these should be
considered to be standard-layout and to have unique object representations.
However, under gcc 7.1, only X is considered to have unique object
representations, and only X and Y are considered to be standard-layout. (It
looks like Y was incorrectly considered to not be standard-layout until PR
80605.)

$ cat t.cc
struct A {};
struct B : A {};
struct C : B {};
struct X : A { char c; };
struct Y : B { char c; };
struct Z : C { char c; };
static_assert(__has_unique_object_representations(X));
static_assert(__has_unique_object_representations(Y));
static_assert(__has_unique_object_representations(Z));
static_assert(__is_standard_layout(X));
static_assert(__is_standard_layout(Y));
static_assert(__is_standard_layout(Z));

$ g++-7 -Wall -Wextra -c t.cc -std=c++17
t.cc:8:1: error: static assertion failed
 static_assert(__has_unique_object_representations(Y));
 ^~~~~~~~~~~~~
t.cc:9:1: error: static assertion failed
 static_assert(__has_unique_object_representations(Z));
 ^~~~~~~~~~~~~
t.cc:12:1: error: static assertion failed
 static_assert(__is_standard_layout(Z));
 ^~~~~~~~~~~~~

$ g++-7 -v
Using built-in specs.
COLLECT_GCC=g++-7
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/7/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 7.1.0-7'
--with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=gcc4-compatible --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --with-system-zlib
--with-target-system-zlib --enable-objc-gc=auto --enable-multiarch
--disable-werror --with-arch-32=i586 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu --disable-initfini-array
Thread model: posix
gcc version 7.1.0 (Debian 7.1.0-7)