https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97566
--- Comment #2 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jason Merrill <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:a4dfd0f089af33f2af57bf422f9859405b9b4a16 commit r11-6918-ga4dfd0f089af33f2af57bf422f9859405b9b4a16 Author: Jason Merrill <ja...@redhat.com> Date: Sun Jan 24 00:55:49 2021 -0500 c++: constexpr and empty fields [PR97566] In the discussion of PR98463, Jakub pointed out that in C++17 and up, cxx_fold_indirect_ref_1 could use the field we build for an empty base. I tried implementing that, but it broke one of the tuple tests, so I did some more digging. To start with, I generalized the PR98463 patch to handle the case where we do have a field, for an empty base or [[no_unique_address]] member. This is enough also for the no-field case because the member of the empty base must itself be an empty field; if it weren't, the base would not be empty. I looked for related PRs and found 97566, which was also fixed by the patch. After some poking around to figure out why, I noticed that the testcase had been breaking because E, though an empty class, has an ABI nvsize of one byte, and we were giving the [[no_unique_address]] FIELD_DECL that DECL_SIZE, whereas in build_base_field_1 empty base fields always get DECL_SIZE zero, and various places were relying on that to recognize empty fields. So I adjusted both the size and the checking. When I adjusted check_bases I wondered if we were correctly handling bases with only empty data members, but it appears we do. I'm deferring the cxx_fold_indirect_ref_1 change until stage 1, as I don't think it actually fixes anything. gcc/cp/ChangeLog: PR c++/97566 PR c++/98463 * class.c (layout_class_type): An empty field gets size 0. (is_empty_field): New. (check_bases): Check it. * cp-tree.h (is_empty_field): Declare it. * constexpr.c (cxx_eval_store_expression): Check it. (cx_check_missing_mem_inits): Likewise. * init.c (perform_member_init): Likewise. * typeck2.c (process_init_constructor_record): Likewise. gcc/testsuite/ChangeLog: PR c++/97566 * g++.dg/cpp2a/no_unique_address10.C: New test. * g++.dg/cpp2a/no_unique_address9.C: New test.
