[Bug c++/94066] [8/9/10 Regression] ICE (starting/ending union member lifetime) in cxx_eval_bare_aggregate, at cp/constexpr.c:3790 since r6-7592

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94066

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:b599bf9d6d1e180d350b71e51e08a66a1bb1546a

commit r10-7313-gb599bf9d6d1e180d350b71e51e08a66a1bb1546a
Author: Patrick Palka <ppalka@redhat.com>
Date:   Thu Mar 19 09:58:28 2020 -0400

    c++: Reject changing active member of union during initialization [PR94066]

    This patch adds a check to detect changing the active union member during
    initialization of another member of the union in cxx_eval_store_expression.
 It
    uses the CONSTRUCTOR_NO_CLEARING flag as a proxy for whether the non-empty
    CONSTRUCTOR of UNION_TYPE we're assigning to is in the process of being
    initialized.

    This patch additionally fixes an issue in reduced_constant_expression_p
where we
    were returning false for an uninitialized union with no active member. 
This
    lets us correctly reject the uninitialized use in the testcase
    testconstexpr-union4.C that we weren't before.

    gcc/cp/ChangeLog:

            PR c++/94066
            * constexpr.c (reduced_constant_expression_p) [CONSTRUCTOR]:
Properly
            handle unions without an initializer.
            (cxx_eval_component_reference): Emit a different diagnostic when
the
            constructor element corresponding to a union member is NULL.
            (cxx_eval_bare_aggregate): When constructing a union, always set
the
            active union member before evaluating the initializer.  Relax
assertion
            that verifies the index of the constructor element we're
initializing
            hasn't been changed.
            (cxx_eval_store_expression): Diagnose changing the active union
member
            while the union is in the process of being initialized.  After
setting
            an active union member, clear CONSTRUCTOR_NO_CLEARING on the
underlying
            CONSTRUCTOR.
            (cxx_eval_constant_expression) [PLACEHOLDER_EXPR]: Don't re-reduce
a
            CONSTRUCTOR returned by lookup_placeholder.

    gcc/testsuite/ChangeLog:

            PR c++/94066
            * g++.dg/cpp1y/constexpr-union2.C: New test.
            * g++.dg/cpp1y/constexpr-union3.C: New test.
            * g++.dg/cpp1y/constexpr-union4.C: New test.
            * g++.dg/cpp1y/constexpr-union5.C: New test.
            * g++.dg/cpp1y/pr94066.C: New test.
            * g++.dg/cpp1y/pr94066-2.C: New test.
            * g++.dg/cpp1y/pr94066-3.C: New test.
            * g++.dg/cpp2a/constexpr-union1.C: New test.