[Bug c++/110382] [13/14 Regression] internal compiler error: in verify_ctor_sanity

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110382

--- Comment #4 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:6e424febfbcb27c21a7fe3a137e614765f9cf9d2

commit r14-2762-g6e424febfbcb27c21a7fe3a137e614765f9cf9d2
Author: Marek Polacek <polacek@redhat.com>
Date:   Fri Jul 21 17:48:37 2023 -0400

    c++: fix ICE with constexpr ARRAY_REF [PR110382]

    This code in cxx_eval_array_reference has been hard to get right.
    In r12-2304 I added some code; in r13-5693 I removed some of it.

    Here the problematic line is "S s = arr[0];" which causes a crash
    on the assert in verify_ctor_sanity:

      gcc_assert (!ctx->object || !DECL_P (ctx->object)
                  || ctx->global->get_value (ctx->object) == ctx->ctor);

    ctx->object is the VAR_DECL 's', which is correct here.  The second
    line points to the problem: we replaced ctx->ctor in
    cxx_eval_array_reference:

      new_ctx.ctor = build_constructor (elem_type, NULL); // #1

    which I think we shouldn't have; the CONSTRUCTOR we created in
    cxx_eval_constant_expression/DECL_EXPR

      new_ctx.ctor = build_constructor (TREE_TYPE (r), NULL);

    had the right type.

    We still need #1 though.  E.g., in constexpr-96241.C, we never
    set ctx.ctor/object before calling cxx_eval_array_reference, so
    we have to build a CONSTRUCTOR there.  And in constexpr-101371-2.C
    we have a ctx.ctor, but it has the wrong type, so we need a new one.

    We can fix the problem by always clearing the object, and, as an
    optimization, only create/free a new ctor when actually needed.

            PR c++/110382

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_array_reference): Create a new constructor
            only when we don't already have a matching one.  Clear the object
            when the type is non-scalar.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-110382.C: New test.