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)
