[Bug c++/97566] [[no_unique_address]] causes miscompiles when mixed with EBO in constexpr context

[cvs-commit at gcc dot gnu.org]

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 <jason@gcc.gnu.org>:

https://gcc.gnu.org/g:a4dfd0f089af33f2af57bf422f9859405b9b4a16

commit r11-6918-ga4dfd0f089af33f2af57bf422f9859405b9b4a16
Author: Jason Merrill <jason@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.