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)