c++: Parenthesized-init of aggregates accepts invalid code [PR94885]

c++: Parenthesized-init of aggregates accepts invalid code [PR94885]

Here we have (conceptually *) something like

  struct B { };
  struct D : B { };
  D(0); // invalid

and in C++20 the ()-initialization has created a { 0 } constructor that
it tries to initialize an object of type D with.  We should reject
initializing an object of type B from 0, but we wrongly accept it because
process_init_constructor_record skips initializers for empty bases/fields:
       if (DECL_SIZE (field) && integer_zerop (DECL_SIZE (field))
           && !TREE_SIDE_EFFECTS (next))
         /* Don't add trivial initialization of an empty base/field to the
            constructor, as they might not be ordered the way the back-end
            expects.  */
         continue;
but here 'next' was error_mark_node, returned by massage_elt_init, so we
wound up with { } which would validly value-initialize the object.

[*] Usually digest_init in build_new_method_call_1 would detect this,
but in this case the instance is is_dummy_object and we don't call
digest just yet.

        PR c++/94885
        * typeck2.c (process_init_constructor_record): Return PICFLAG_ERRONEOUS
        if an initializer element was erroneous.

        * g++.dg/cpp2a/paren-init26.C: New test.