[Bug c++/94074] [10 Regression] bogus modifying a const object error with const COMPONENT_REF

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94074

--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
commit 7eb5be6ab91ec03f93038ac2bcf3028cf2e7c82b
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Mar 6 17:30:11 2020 -0500

    c++: Fix wrong modifying const object error for COMPONENT_REF [PR94074]

    I got a report that building Chromium fails with the "modifying a const
    object" error.  After some poking I realized it's a bug in GCC, not in
    their codebase.

    Much like with ARRAY_REFs, which can be const even though the array
    itself isn't, COMPONENT_REFs can be const although neither the object
    nor the field were declared const.  So let's dial down the checking.
    Here the COMPONENT_REF was const because of the "const_cast<const U &>(m)"
    thing -- cxx_eval_component_reference then builds a COMPONENT_REF with
    TREE_TYPE (t).

    While looking into this I noticed that we don't detect modifying a const
    object in certain cases like in
    <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94074#c2>.  That's because
    we never evaluate an X::X() CALL_EXPR -- there's none.  Fixed as per
    Jason's suggestion by setting TREE_READONLY on a CONSTRUCTOR after
    initialization in cxx_eval_store_expression.

    2020-03-11  Marek Polacek  <polacek@redhat.com>
                Jason Merrill  <jason@redhat.com>

            PR c++/94074 - wrong modifying const object error for
COMPONENT_REF.
            * constexpr.c (cref_has_const_field): New function.
            (modifying_const_object_p): Consider a COMPONENT_REF
            const only if any of its fields are const.
            (cxx_eval_store_expression): Mark a CONSTRUCTOR of a const type
            as readonly after its initialization has been done.

            * g++.dg/cpp1y/constexpr-tracking-const17.C: New test.
            * g++.dg/cpp1y/constexpr-tracking-const18.C: New test.
            * g++.dg/cpp1y/constexpr-tracking-const19.C: New test.
            * g++.dg/cpp1y/constexpr-tracking-const20.C: New test.
            * g++.dg/cpp1y/constexpr-tracking-const21.C: New test.
            * g++.dg/cpp1y/constexpr-tracking-const22.C: New test.