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)
Confirmed.
Other no_unique_address ABI issues: PR96052, PR95976
The releases/gcc-10 branch has been updated by Jason Merrill <jason@gcc.gnu.org>: https://gcc.gnu.org/g:3bb330022ce47a3e8966a9930f392e497c608f59 commit r10-8456-g3bb330022ce47a3e8966a9930f392e497c608f59 Author: Jason Merrill <jason@redhat.com> Date: Thu Jul 9 15:11:12 2020 -0400 c++: [[no_unique_address]] fixes. [PR96105] We were wrongly checking is_empty_class on the result of strip_array_types rather than the actual field type. We weren't considering the alignment of the data member. We needed to handle unions the same way as layout_nonempty_base_or_field. gcc/cp/ChangeLog: PR c++/96105 PR c++/96052 PR c++/95976 * class.c (check_field_decls): An array of empty classes is not an empty data member. (layout_empty_base_or_field): Handle explicit alignment. Fix union handling. gcc/testsuite/ChangeLog: PR c++/96105 PR c++/96052 PR c++/95976 * g++.dg/cpp2a/no_unique_address4.C: New test. * g++.dg/cpp2a/no_unique_address5.C: New test. * g++.dg/cpp2a/no_unique_address6.C: New test.
The master branch has been updated by Jason Merrill <jason@gcc.gnu.org>: https://gcc.gnu.org/g:e47dfca5aa473e77fdff95d631dc39de87a41eec commit r11-2014-ge47dfca5aa473e77fdff95d631dc39de87a41eec Author: Jason Merrill <jason@redhat.com> Date: Thu Jul 9 15:11:12 2020 -0400 c++: [[no_unique_address]] fixes. [PR96105] We were wrongly checking is_empty_class on the result of strip_array_types rather than the actual field type. We weren't considering the alignment of the data member. We needed to handle unions the same way as layout_nonempty_base_or_field. gcc/cp/ChangeLog: PR c++/96105 PR c++/96052 PR c++/95976 * class.c (check_field_decls): An array of empty classes is not an empty data member. (layout_empty_base_or_field): Handle explicit alignment. Fix union handling. gcc/testsuite/ChangeLog: PR c++/96105 PR c++/96052 PR c++/95976 * g++.dg/cpp2a/no_unique_address4.C: New test. * g++.dg/cpp2a/no_unique_address5.C: New test. * g++.dg/cpp2a/no_unique_address6.C: New test.
Fixed for 10.2/11.