[Bug c++/105491] [10/11/12 Regression] Usage of __constinit with -std=c++11 does is rejected

["cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105491

--- Comment #10 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-12 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:e30b73bad9486f11b6b0022ae4a3edfc0f9da4bb

commit r12-8445-ge30b73bad9486f11b6b0022ae4a3edfc0f9da4bb
Author: Patrick Palka <ppalka@redhat.com>
Date:   Wed Jun 1 08:47:25 2022 -0400

    c++: constexpr init of union sub-aggr w/ base [PR105491]

    Here ever since r10-7313-gb599bf9d6d1e18, reduced_constant_expression_p
    in C++11/14 is rejecting the marked sub-aggregate initializer (of type S)

      W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}};
                         ^
    ultimately because said initializer has CONSTRUCTOR_NO_CLEARING set,
    hence the function must verify that all fields of S are initialized.
    And before C++17 it doesn't expect to see base class fields (since
    next_initializable_field skips over them), so the presence thereof
    causes r_c_e_p to return false.

    The reason r10-7313-gb599bf9d6d1e18 causes this is because in that
    commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether
    we're in middle of activating a union member.  This ends up affecting
    clear_no_implicit_zero, which recurses into sub-aggregate initializers
    only if the outer initializer has CONSTRUCTOR_NO_CLEARING set.  After
    that commit, the outer union initializer above no longer has the flag
    set at this point and so clear_no_implicit_zero no longer recurses into
    the marked inner initializer.

    But arguably r_c_e_p should be able to accept the marked initializer
    regardless of whether CONSTRUCTOR_NO_CLEARING is set.  The primary bug
    therefore seems to be that r_c_e_p relies on next_initializable_field
    which skips over base class fields in C++11/14.  To fix this, this patch
    introduces a new helper function next_subobject_field which is like
    next_initializable_field except that it never skips base class fields,
    and makes r_c_e_p use it.  This patch then renames next_initializable_field
    to next_aggregate_field (and makes it skip over vptr fields again).

    NB: This minimal backport of r13-211-g0c7bce0ac184c0 for 12.2 just adds
    next_subobject_field and makes reduced_constant_expression_p use it.

            PR c++/105491

    gcc/cp/ChangeLog:

            * constexpr.cc (reduced_constant_expression_p): Use
            next_subobject_field instead.
            * cp-tree.h (next_subobject_field): Declare.
            * decl.cc (next_subobject_field): Define.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp0x/constexpr-union7.C: New test.
            * g++.dg/cpp0x/constexpr-union7a.C: New test.
            * g++.dg/cpp2a/constinit17.C: New test.

    (cherry picked from commit 0c7bce0ac184c057bacad9c8e615ce82923835fd)