[Bug c++/101371] [9/10/11 Regression] constexpr expansions trigger internal Compiler Error

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101371

--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-11 branch has been updated by Marek Polacek
<mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:da47a84e277c7fbeebbf9c1e7dc1e8ba3277fe53

commit r11-9710-gda47a84e277c7fbeebbf9c1e7dc1e8ba3277fe53
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Jul 13 17:16:54 2021 -0400

    c++: constexpr array reference and value-initialization [PR101371]

    This PR gave me a hard time: I saw multiple issues starting with
    different revisions.  But ultimately the root cause seems to be
    the following, and the attached patch fixes all issues I've found
    here.

    In cxx_eval_array_reference we create a new constexpr context for the
    CP_AGGREGATE_TYPE_P case, but we also have to create it for the
    non-aggregate case.  In this test, we are evaluating

      ((B *)this)->a = rhs->a

    which means that we set ctx.object to ((B *)this)->a.  Then we proceed
    to evaluate the initializer, rhs->a.  For *rhs, we eval rhs, a PARM_DECL,
    for which we have (const B &) &c.arr[0] in the hash table.  Then
    cxx_fold_indirect_ref gives us c.arr[0].  c is evaluated to {.arr={}} so
    c.arr is {}.  Now we want c.arr[0], so we end up in
cxx_eval_array_reference
    and since we're initializing from {}, we call build_value_init which
    gives us an AGGR_INIT_EXPR that calls 'constexpr B::B()'.  Then we
    evaluate this AGGR_INIT_EXPR and since its first argument is dummy,
    we take ctx.object instead.  But that is the wrong object, we're not
    initializing ((B *)this)->a here.  And so we wound up with an
    initializer for A, and then crash in cxx_eval_component_reference:

      gcc_assert (DECL_CONTEXT (part) == TYPE_MAIN_VARIANT (TREE_TYPE
(whole)));

    where DECL_CONTEXT (part) is B (as it should be) but the type of whole
    was A.

    So create a new object, if there already was one, and the element type
    is not a scalar.

            PR c++/101371

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_eval_array_reference): Create a new .object
            and .ctor for the non-aggregate non-scalar case too when
            value-initializing.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-101371-2.C: New test.
            * g++.dg/cpp1y/constexpr-101371.C: New test.

    (cherry picked from commit a42f8120442cf3ba25d621bed857b5be19019d0c)