[Bug c++/108158] [11/12/13 Regression] modification of '...' is not a constant expression since r12-2304

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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108158

--- Comment #6 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:27ac6a707e7438c3cec79c24f5d53de79493e2f8

commit r13-5693-g27ac6a707e7438c3cec79c24f5d53de79493e2f8
Author: Marek Polacek <polacek@redhat.com>
Date:   Mon Jan 30 09:02:00 2023 -0500

    c++: wrong error with constexpr array and value-init [PR108158]

    In this test case, we find ourselves evaluating 't' which is
    ((const struct carray *) this)->data_[VIEW_CONVERT_EXPR<long int>(index)]
    in cxx_eval_array_reference.  ctx->object is non-null, a RESULT_DECL, so
    we replace it with 't':

      new_ctx.object = t; // result_decl replaced

    and then we go to cxx_eval_constant_expression to evaluate an
    AGGR_INIT_EXPR, where we end up evaluating an INIT_EXPR (which is in the
    body of the constructor for seed_or_index):

      ((struct seed_or_index *) this)->value_ = NON_LVALUE_EXPR <0>

    whereupon in cxx_eval_store_expression we go to the probe loop
    where the 'this' is evaluated to

      ze_set.tables_.first_table_.data_[0]

    so the 'object' is ze_set, but that isn't in ctx->global->get_value_ptr
    so we fail with a bogus error.  ze_set is not there because it comes
    from a different constexpr context (it's not in cv_cache either).

    The problem started with r12-2304 where I added the new_ctx.object
    replacement.  That was to prevent a type mismatch: the type of 't'
    and ctx.object were different.

    It seems clear that we shouldn't have replaced ctx.object here.
    The cxx_eval_array_reference I mentioned earlier is called from
    cxx_eval_store_expression:
     6257       init = cxx_eval_constant_expression (&new_ctx, init,
vc_prvalue,
     6258                                            non_constant_p,
overflow_p);
    which already created a new context, whose .object we should be
    using unless, for instance, INIT contained a.b and we're evaluating
    the 'a' part, which I think was the case for r12-2304; in that case
    ctx.object has to be something different.

    It no longer seems necessary to replace new_ctx.object (likely due to
    changes in empty class handling).

            PR c++/108158

    gcc/cp/ChangeLog:

            * constexpr.cc (cxx_eval_array_reference): Don't replace
            new_ctx.object.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-108158.C: New test.