Re: [PATCH] c++: constexpr init of union sub-aggr w/ base [PR105491]

[Patrick Palka <ppalka@redhat.com>]

On Sat, May 7, 2022 at 5:18 PM Jason Merrill <jason@redhat.com> wrote:
>
> On 5/6/22 16:46, Patrick Palka wrote:
> > On Fri, 6 May 2022, Jason Merrill wrote:
> >
> >> On 5/6/22 16:10, Patrick Palka wrote:
> >>> On Fri, 6 May 2022, Patrick Palka wrote:
> >>>
> >>>> On Fri, 6 May 2022, Jason Merrill wrote:
> >>>>
> >>>>> On 5/6/22 14:00, Patrick Palka wrote:
> >>>>>> On Fri, 6 May 2022, Patrick Palka wrote:
> >>>>>>
> >>>>>>> On Fri, 6 May 2022, Jason Merrill wrote:
> >>>>>>>
> >>>>>>>> On 5/6/22 11:22, Patrick Palka wrote:
> >>>>>>>>> Here ever since r10-7313-gb599bf9d6d1e18,
> >>>>>>>>> reduced_constant_expression_p
> >>>>>>>>> in C++11/14 is rejecting the marked sub-aggregate initializer
> >>>>>>>>> (of type
> >>>>>>>>> S)
> >>>>>>>>>
> >>>>>>>>>       W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}}
> >>>>>>>>>                          ^
> >>>>>>>>>
> >>>>>>>>> ultimately because said initializer has CONSTRUCTOR_NO_CLEARING
> >>>>>>>>> set,
> >>>>>>>>> and
> >>>>>>>>> so the function proceeds to verify that all fields of S are
> >>>>>>>>> initialized.
> >>>>>>>>> And before C++17 we don't expect to see base class fields (since
> >>>>>>>>> next_initializable_field skips over the), so the base class
> >>>>>>>>> initializer
> >>>>>>>>> causes r_c_e_p to return false.
> >>>>>>>>
> >>>>>>>> That seems like the primary bug.  I guess r_c_e_p shouldn't be
> >>>>>>>> using
> >>>>>>>> next_initializable_field.  Really that function should only be
> >>>>>>>> used for
> >>>>>>>> aggregates.
> >>>>>>>
> >>>>>>> I see, I'll try replacing it in r_c_e_p.  Would that be in addition
> >>>>>>> to
> >>>>>>> or instead of the clear_no_implicit_zero approach?
> >>>>>>
> >>>>>> I'm testing the following, which uses a custom predicate instead of
> >>>>>> next_initializable_field in r_c_e_p.
> >>>>>
> >>>>> Let's make it a public predicate, not internal to r_c_e_p.  Maybe it
> >>>>> could be
> >>>>> next_subobject_field, and the current next_initializable_field change to
> >>>>> next_aggregate_field?
> >>>>
> >>>> Will do.
> >>>>
> >>>>>
> >>>>>> Looks like the inner initializer {.D.2387={.m=0}, .b=0} is formed
> >>>>>> during
> >>>>>> the subobject constructor call:
> >>>>>>
> >>>>>>     V::V (&((struct S *) this)->D.2120);
> >>>>>>
> >>>>>> after the evaluation of which, 'result' in cxx_eval_call_expression is
> >>>>>> NULL
> >>>>>> (presumably because it's a CALL_EXPR, not AGGR_INIT_EXPR?):
> >>>>>>
> >>>>>>     /* This can be null for a subobject constructor call, in
> >>>>>>        which case what we care about is the initialization
> >>>>>>        side-effects rather than the value.  We could get at the
> >>>>>>        value by evaluating *this, but we don't bother; there's
> >>>>>>        no need to put such a call in the hash table.  */
> >>>>>>     result = lval ? ctx->object : ctx->ctor;
> >>>>>>
> >>>>>> so we end up not calling clear_no_implicit_zero for the inner
> >>>>>> initializer
> >>>>>> directly.  We only call clear_no_implicit_zero after evaluating the
> >>>>>> AGGR_INIT_EXPR for outermost initializer (of type W).
> >>>>>
> >>>>> Maybe for constructors we could call it on ctx->ctor instead of result,
> >>>>> or
> >>>>> call r_c_e_p in C++20+?
> >>>>
> >>>> But both ctx->ctor and ->object are NULL during a subobject constructor
> >>>> call (since we apparently clear these fields when entering a
> >>>> STATEMENT_LIST):
> >>>>
> >>>> So I tried instead obtaining the constructor by evaluating new_obj via
> >>>>
> >>>> --- a/gcc/cp/constexpr.cc
> >>>> +++ b/gcc/cp/constexpr.cc
> >>>> @@ -2993,6 +2988,9 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
> >>>> tree t,
> >>>>         in order to detect reading an unitialized object in constexpr
> >>>> instead
> >>>>         of value-initializing it.  (reduced_constant_expression_p is
> >>>> expected to
> >>>>         take care of clearing the flag.)  */
> >>>> +  if (new_obj && DECL_CONSTRUCTOR_P (fun))
> >>>> +    result = cxx_eval_constant_expression (ctx, new_obj, /*lval=*/false,
> >>>> +                                          non_constant_p, overflow_p);
> >>>>      if (TREE_CODE (result) == CONSTRUCTOR
> >>>>          && (cxx_dialect < cxx20
> >>>>             || !DECL_CONSTRUCTOR_P (fun)))
> >>>>
> >>>> but that seems to break e.g. g++.dg/cpp2a/constexpr-init12.C because
> >>>> after the subobject constructor call
> >>>>
> >>>>     S::S (&((struct W *) this)->s, NON_LVALUE_EXPR <8>);
> >>>>
> >>>> the constructor for the subobject a.s in new_obj is still completely
> >>>> missing (I suppose because S::S doesn't initialize any of its members)
> >>>> so trying to obtain it causes us to complain too soon from
> >>>> cxx_eval_component_reference:
> >>>>
> >>>> constexpr-init12.C:16:24:   in ‘constexpr’ expansion of ‘W(42)’
> >>>> constexpr-init12.C:10:22:   in ‘constexpr’ expansion of
> >>>> ‘((W*)this)->W::s.S::S(8)’
> >>>> constexpr-init12.C:16:24: error: accessing uninitialized member ‘W::s’
> >>>>      16 | constexpr auto a = W(42); // { dg-error "not a constant
> >>>> expression" }
> >>>>         |                        ^
> >>>>
> >>>>>
> >>>>> It does seem dubious that we would clear the flag on an outer ctor when
> >>>>> it's
> >>>>> still set on an inner ctor, should probably add an assert somewhere.
> >>>>
> >>>> Makes sense, not sure where the best place would be..
> >>>
> >>> On second thought, if I'm understanding your suggestion correctly, I
> >>> don't think we can generally enforce such a property for
> >>> CONSTRUCTOR_NO_CLEARING, given how cxx_eval_store_expression uses it for
> >>> unions:
> >>>
> >>>     union U {
> >>>       struct { int x, y; } a;
> >>>     } u;
> >>>     u.a.x = 0;
> >>>
> >>> Here after evaluating the assignment, the outer ctor for the union will
> >>> have CONSTRUCTOR_NO_CLEARING cleared to indicate we finished activating
> >>> the union member, but the inner ctor is certainly not fully initialized
> >>> so it'll have CONSTRUCTOR_NO_CLEARING set still.
> >>
> >> Why clear the flag on the union before the inner ctor is fully initialized, if
> >> the intent is to prevent changing the active member during initialization?
> >>
> >> In the loop over ctors in cxx_eval_store_expression, I'd think if we encounter
> >> a CONSTRUCTOR_NO_CLEARING ctor along the way, we shouldn't clear the flag on
> >> an outer ctor.
> >
> > Wouldn't that prevent us from later activating a different member, which is
> > allowed in C++20?  It would cause us to reject e.g.
> >
> >      constexpr auto f() {
> >        union U {
> >          struct { int x, y; } a;
> >          int b;
> >        } u;
> >        u.a.x = 0;
> >        u.b = 0;
> >        return u;
> >      }
> >
> >      static_assert(f().b == 0);
> >
> > even in C++20 with
> >
> > <source>:7:7: error: change of the active member of a union from ‘f()::U::a’ to ‘f()::U::b’ during initialization
> >      7 |   u.b = 0;
> >        |   ~~~~^~~
> >
> > because when evaluating the second assignment we'd see that the
> > CONSTRUCTOR_NO_CLEARING flag is still set on the union while it already
> > has an activated member.
>
> Ah, makes sense.
>
> > On a related note, here's the patch that mechanically renames
> > next_initializable_field to next_aggregate_field and makes it skip over vptr
> > fields, and defines next_subobject_field alongside it for use by r_c_e_p.
> > Bootstrap and regtesting in progress.
>
> OK if successful.

Thanks, committed as r13-211-g0c7bce0ac184c0.  Would this patch be
suitable for backporting to 12?  It seems safe enough.  Alternatively
I guess we could backport the original workaround that tweaks
clear_no_implicit_zero.

>
> >
> > -- >8 --
> >
> >       PR c++/105491
> >
> > gcc/cp/ChangeLog:
> >
> >       * call.cc (field_in_pset): Adjust after next_initializable_field
> >       renaming.
> >       (build_aggr_conv): Likewise.
> >       (convert_like_internal): Likewise.
> >       (type_has_extended_temps): Likewise.
> >       * class.cc (default_init_uninitialized_part): Likewise.
> >       (finish_struct): Likewise.
> >       * constexpr.cc (cx_check_missing_mem_inits): Likewise.
> >       (reduced_constant_expression_p): Use next_subobject_field
> >       instead.
> >       * cp-gimplify.cc (get_source_location_impl_type): Adjust after
> >       next_initializable_field renaming.
> >       (fold_builtin_source_location): Likewise.
> >       * cp-tree.h (next_initializable_field): Rename to ...
> >       (next_aggregate_field): ... this.
> >       (next_subobject_field): Declare.
> >       * decl.cc (next_aggregate_field): Renamed from ...
> >       (next_initializable_field): ... this.  Skip over vptr fields.
> >       Document that this is now intended for aggregate classes.
> >       (next_subobject_field): Define.
> >       (reshape_init_class): Adjust after next_initializable_field
> >       renaming.
> >       * init.cc (build_value_init_noctor): Likewise.
> >       (emit_mem_initializers): Likewise.
> >       * lambda.cc (build_capture_proxy): Likewise.
> >       * method.cc (build_comparison_op): Likewise.
> >       * pt.cc (maybe_aggr_guide): Likewise.
> >       * tree.cc (structural_type_p): Likewise.
> >       * typeck2.cc (split_nonconstant_init_1): Likewise.
> >       (digest_init_r): Likewise.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C: New test.
> >       * gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C: New test.
> >       * gcc/testsuite/g++.dg/cpp2a/constinit17.C: New test.
> > ---
> >   gcc/cp/call.cc                                | 14 +++----
> >   gcc/cp/class.cc                               |  8 ++--
> >   gcc/cp/constexpr.cc                           | 10 ++---
> >   gcc/cp/cp-gimplify.cc                         |  4 +-
> >   gcc/cp/cp-tree.h                              |  3 +-
> >   gcc/cp/decl.cc                                | 38 +++++++++++++------
> >   gcc/cp/init.cc                                |  6 +--
> >   gcc/cp/lambda.cc                              |  4 +-
> >   gcc/cp/method.cc                              |  8 ++--
> >   gcc/cp/pt.cc                                  |  2 +-
> >   gcc/cp/tree.cc                                |  4 +-
> >   gcc/cp/typeck2.cc                             |  4 +-
> >   gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C | 17 +++++++++
> >   .../g++.dg/cpp0x/constexpr-union7a.C          | 15 ++++++++
> >   gcc/testsuite/g++.dg/cpp2a/constinit17.C      | 24 ++++++++++++
> >   15 files changed, 117 insertions(+), 44 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C
> >   create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C
> >   create mode 100644 gcc/testsuite/g++.dg/cpp2a/constinit17.C
> >
> > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > index 959279d6216..0240e364324 100644
> > --- a/gcc/cp/call.cc
> > +++ b/gcc/cp/call.cc
> > @@ -948,7 +948,7 @@ field_in_pset (hash_set<tree, true> &pset, tree field)
> >       for (field = TYPE_FIELDS (TREE_TYPE (field));
> >        field; field = DECL_CHAIN (field))
> >         {
> > -     field = next_initializable_field (field);
> > +     field = next_aggregate_field (field);
> >       if (field == NULL_TREE)
> >         break;
> >       if (field_in_pset (pset, field))
> > @@ -965,7 +965,7 @@ build_aggr_conv (tree type, tree ctor, int flags, tsubst_flags_t complain)
> >   {
> >     unsigned HOST_WIDE_INT i = 0;
> >     conversion *c;
> > -  tree field = next_initializable_field (TYPE_FIELDS (type));
> > +  tree field = next_aggregate_field (TYPE_FIELDS (type));
> >     tree empty_ctor = NULL_TREE;
> >     hash_set<tree, true> pset;
> >
> > @@ -1011,7 +1011,7 @@ build_aggr_conv (tree type, tree ctor, int flags, tsubst_flags_t complain)
> >       }
> >       }
> >
> > -  for (; field; field = next_initializable_field (DECL_CHAIN (field)))
> > +  for (; field; field = next_aggregate_field (DECL_CHAIN (field)))
> >       {
> >         tree ftype = TREE_TYPE (field);
> >         tree val;
> > @@ -8098,10 +8098,10 @@ convert_like_internal (conversion *convs, tree expr, tree fn, int argnum,
> >       totype = complete_type_or_maybe_complain (totype, NULL_TREE, complain);
> >       if (!totype)
> >         return error_mark_node;
> > -     tree field = next_initializable_field (TYPE_FIELDS (totype));
> > +     tree field = next_aggregate_field (TYPE_FIELDS (totype));
> >       vec<constructor_elt, va_gc> *vec = NULL;
> >       CONSTRUCTOR_APPEND_ELT (vec, field, array);
> > -     field = next_initializable_field (DECL_CHAIN (field));
> > +     field = next_aggregate_field (DECL_CHAIN (field));
> >       CONSTRUCTOR_APPEND_ELT (vec, field, size_int (len));
> >       tree new_ctor = build_constructor (totype, vec);
> >       return get_target_expr_sfinae (new_ctor, complain);
> > @@ -13267,8 +13267,8 @@ type_has_extended_temps (tree type)
> >       {
> >         if (is_std_init_list (type))
> >       return true;
> > -      for (tree f = next_initializable_field (TYPE_FIELDS (type));
> > -        f; f = next_initializable_field (DECL_CHAIN (f)))
> > +      for (tree f = next_aggregate_field (TYPE_FIELDS (type));
> > +        f; f = next_aggregate_field (DECL_CHAIN (f)))
> >       if (type_has_extended_temps (TREE_TYPE (f)))
> >         return true;
> >       }
> > diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
> > index bc94ed45e17..3c195b35396 100644
> > --- a/gcc/cp/class.cc
> > +++ b/gcc/cp/class.cc
> > @@ -5494,8 +5494,8 @@ default_init_uninitialized_part (tree type)
> >         if (r)
> >       return r;
> >       }
> > -  for (t = next_initializable_field (TYPE_FIELDS (type)); t;
> > -       t = next_initializable_field (DECL_CHAIN (t)))
> > +  for (t = next_aggregate_field (TYPE_FIELDS (type)); t;
> > +       t = next_aggregate_field (DECL_CHAIN (t)))
> >       if (!DECL_INITIAL (t) && !DECL_ARTIFICIAL (t))
> >         {
> >       r = default_init_uninitialized_part (TREE_TYPE (t));
> > @@ -7781,10 +7781,10 @@ finish_struct (tree t, tree attributes)
> >         bool ok = false;
> >         if (processing_template_decl)
> >       {
> > -       tree f = next_initializable_field (TYPE_FIELDS (t));
> > +       tree f = next_aggregate_field (TYPE_FIELDS (t));
> >         if (f && TYPE_PTR_P (TREE_TYPE (f)))
> >           {
> > -           f = next_initializable_field (DECL_CHAIN (f));
> > +           f = next_aggregate_field (DECL_CHAIN (f));
> >             if (f && same_type_p (TREE_TYPE (f), size_type_node))
> >               ok = true;
> >           }
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 9b1e71857fc..24717b4f8d5 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -784,7 +784,7 @@ cx_check_missing_mem_inits (tree ctype, tree body, bool complain)
> >
> >     if (TREE_CODE (ctype) == UNION_TYPE)
> >       {
> > -      if (nelts == 0 && next_initializable_field (field))
> > +      if (nelts == 0 && next_aggregate_field (field))
> >       {
> >         if (complain)
> >           error ("%<constexpr%> constructor for union %qT must "
> > @@ -3053,7 +3053,7 @@ reduced_constant_expression_p (tree t)
> >             field = NULL_TREE;
> >           }
> >         else
> > -         field = next_initializable_field (TYPE_FIELDS (TREE_TYPE (t)));
> > +         field = next_subobject_field (TYPE_FIELDS (TREE_TYPE (t)));
> >       }
> >         else
> >       field = NULL_TREE;
> > @@ -3065,15 +3065,15 @@ reduced_constant_expression_p (tree t)
> >           return false;
> >         /* Empty class field may or may not have an initializer.  */
> >         for (; field && e.index != field;
> > -            field = next_initializable_field (DECL_CHAIN (field)))
> > +            field = next_subobject_field (DECL_CHAIN (field)))
> >           if (!is_really_empty_class (TREE_TYPE (field),
> >                                       /*ignore_vptr*/false))
> >             return false;
> >         if (field)
> > -         field = next_initializable_field (DECL_CHAIN (field));
> > +         field = next_subobject_field (DECL_CHAIN (field));
> >       }
> >         /* There could be a non-empty field at the end.  */
> > -      for (; field; field = next_initializable_field (DECL_CHAIN (field)))
> > +      for (; field; field = next_subobject_field (DECL_CHAIN (field)))
> >       if (!is_really_empty_class (TREE_TYPE (field), /*ignore_vptr*/false))
> >         return false;
> >   ok:
> > diff --git a/gcc/cp/cp-gimplify.cc b/gcc/cp/cp-gimplify.cc
> > index b52d9cb5754..ee47a4bfd37 100644
> > --- a/gcc/cp/cp-gimplify.cc
> > +++ b/gcc/cp/cp-gimplify.cc
> > @@ -3106,7 +3106,7 @@ get_source_location_impl_type (location_t loc)
> >
> >     int cnt = 0;
> >     for (tree field = TYPE_FIELDS (type);
> > -       (field = next_initializable_field (field)) != NULL_TREE;
> > +       (field = next_aggregate_field (field)) != NULL_TREE;
> >          field = DECL_CHAIN (field))
> >       {
> >         if (DECL_NAME (field) != NULL_TREE)
> > @@ -3281,7 +3281,7 @@ fold_builtin_source_location (location_t loc)
> >         vec<constructor_elt, va_gc> *v = NULL;
> >         vec_alloc (v, 4);
> >         for (tree field = TYPE_FIELDS (source_location_impl);
> > -        (field = next_initializable_field (field)) != NULL_TREE;
> > +        (field = next_aggregate_field (field)) != NULL_TREE;
> >          field = DECL_CHAIN (field))
> >       {
> >         const char *n = IDENTIFIER_POINTER (DECL_NAME (field));
> > diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> > index 663fe7a20fc..abcef7c20c4 100644
> > --- a/gcc/cp/cp-tree.h
> > +++ b/gcc/cp/cp-tree.h
> > @@ -6870,7 +6870,8 @@ extern bool is_direct_enum_init                 (tree, tree);
> >   extern void initialize_artificial_var               (tree, vec<constructor_elt, va_gc> *);
> >   extern tree check_var_type                  (tree, tree, location_t);
> >   extern tree reshape_init                        (tree, tree, tsubst_flags_t);
> > -extern tree next_initializable_field (tree);
> > +extern tree next_aggregate_field             (tree);
> > +extern tree next_subobject_field             (tree);
> >   extern tree first_field                             (const_tree);
> >   extern tree fndecl_declared_return_type             (tree);
> >   extern bool undeduced_auto_decl                     (tree);
> > diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> > index c9110db796a..d5ab2d0ad87 100644
> > --- a/gcc/cp/decl.cc
> > +++ b/gcc/cp/decl.cc
> > @@ -6382,22 +6382,38 @@ struct reshape_iter
> >
> >   static tree reshape_init_r (tree, reshape_iter *, tree, tsubst_flags_t);
> >
> > +/* FIELD is an element of TYPE_FIELDS of an aggregate class, or NULL.  In the
> > +   former case, the value returned is the next FIELD_DECL (possibly FIELD
> > +   itself) that can be initialized.  If there are no more such fields, the
> > +   return value will be NULL.  */
> > +
> > +tree
> > +next_aggregate_field (tree field)
> > +{
> > +  while (field
> > +      && (TREE_CODE (field) != FIELD_DECL
> > +          || DECL_UNNAMED_BIT_FIELD (field)
> > +          || (DECL_ARTIFICIAL (field)
> > +              /* In C++17, aggregates can have base classes.  */
> > +              && !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field)))))
> > +    field = DECL_CHAIN (field);
> > +
> > +  return field;
> > +}
> > +
> >   /* FIELD is an element of TYPE_FIELDS or NULL.  In the former case, the value
> > -   returned is the next FIELD_DECL (possibly FIELD itself) that can be
> > -   initialized.  If there are no more such fields, the return value
> > -   will be NULL.  */
> > +   returned is the next FIELD_DECL (possibly FIELD itself) that corresponds
> > +   to a subobject.  If there are no more such fields, the return value will be
> > +   NULL.  */
> >
> >   tree
> > -next_initializable_field (tree field)
> > +next_subobject_field (tree field)
> >   {
> >     while (field
> >        && (TREE_CODE (field) != FIELD_DECL
> >            || DECL_UNNAMED_BIT_FIELD (field)
> >            || (DECL_ARTIFICIAL (field)
> > -              /* In C++17, don't skip base class fields.  */
> > -              && !(cxx_dialect >= cxx17 && DECL_FIELD_IS_BASE (field))
> > -              /* Don't skip vptr fields.  We might see them when we're
> > -                 called from reduced_constant_expression_p.  */
> > +              && !DECL_FIELD_IS_BASE (field)
> >                && !DECL_VIRTUAL_P (field))))
> >       field = DECL_CHAIN (field);
> >
> > @@ -6595,7 +6611,7 @@ reshape_init_class (tree type, reshape_iter *d, bool first_initializer_p,
> >     if (base_binfo)
> >       field = base_binfo;
> >     else
> > -    field = next_initializable_field (TYPE_FIELDS (type));
> > +    field = next_aggregate_field (TYPE_FIELDS (type));
> >
> >     if (!field)
> >       {
> > @@ -6760,10 +6776,10 @@ reshape_init_class (tree type, reshape_iter *d, bool first_initializer_p,
> >         if (BINFO_BASE_ITERATE (binfo, ++binfo_idx, base_binfo))
> >           field = base_binfo;
> >         else
> > -         field = next_initializable_field (TYPE_FIELDS (type));
> > +         field = next_aggregate_field (TYPE_FIELDS (type));
> >       }
> >         else
> > -     field = next_initializable_field (DECL_CHAIN (field));
> > +     field = next_aggregate_field (DECL_CHAIN (field));
> >       }
> >
> >     /* A trailing aggregate element that is a pack expansion is assumed to
> > diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> > index 75ab965a218..f1ed9336dc9 100644
> > --- a/gcc/cp/init.cc
> > +++ b/gcc/cp/init.cc
> > @@ -422,7 +422,7 @@ build_value_init_noctor (tree type, tsubst_flags_t complain)
> >                 && !COMPLETE_TYPE_P (ftype)
> >                 && !TYPE_DOMAIN (ftype)
> >                 && COMPLETE_TYPE_P (TREE_TYPE (ftype))
> > -               && (next_initializable_field (DECL_CHAIN (field))
> > +               && (next_aggregate_field (DECL_CHAIN (field))
> >                     == NULL_TREE))
> >               continue;
> >
> > @@ -1477,9 +1477,9 @@ emit_mem_initializers (tree mem_inits)
> >
> >     /* Initially that is all of them.  */
> >     if (warn_uninitialized)
> > -    for (tree f = next_initializable_field (TYPE_FIELDS (current_class_type));
> > +    for (tree f = next_aggregate_field (TYPE_FIELDS (current_class_type));
> >        f != NULL_TREE;
> > -      f = next_initializable_field (DECL_CHAIN (f)))
> > +      f = next_aggregate_field (DECL_CHAIN (f)))
> >         if (!DECL_ARTIFICIAL (f)
> >         && !is_really_empty_class (TREE_TYPE (f), /*ignore_vptr*/false))
> >       uninitialized.add (f);
> > diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
> > index afac53b6d7c..27704a578af 100644
> > --- a/gcc/cp/lambda.cc
> > +++ b/gcc/cp/lambda.cc
> > @@ -425,9 +425,9 @@ build_capture_proxy (tree member, tree init)
> >     if (DECL_VLA_CAPTURE_P (member))
> >       {
> >         /* Rebuild the VLA type from the pointer and maxindex.  */
> > -      tree field = next_initializable_field (TYPE_FIELDS (type));
> > +      tree field = next_aggregate_field (TYPE_FIELDS (type));
> >         tree ptr = build_simple_component_ref (object, field);
> > -      field = next_initializable_field (DECL_CHAIN (field));
> > +      field = next_aggregate_field (DECL_CHAIN (field));
> >         tree max = build_simple_component_ref (object, field);
> >         type = build_cplus_array_type (TREE_TYPE (TREE_TYPE (ptr)),
> >                                    build_index_type (max));
> > diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
> > index 903ee666ef3..0dffd648b0b 100644
> > --- a/gcc/cp/method.cc
> > +++ b/gcc/cp/method.cc
> > @@ -1465,7 +1465,7 @@ build_comparison_op (tree fndecl, bool defining, tsubst_flags_t complain)
> >     /* A defaulted comparison operator function for class C is defined as
> >        deleted if ... C has variant members.  */
> >     if (TREE_CODE (ctype) == UNION_TYPE
> > -      && next_initializable_field (TYPE_FIELDS (ctype)))
> > +      && next_aggregate_field (TYPE_FIELDS (ctype)))
> >       {
> >         if (complain & tf_error)
> >       inform (info.loc, "cannot default compare union %qT", ctype);
> > @@ -1518,9 +1518,9 @@ build_comparison_op (tree fndecl, bool defining, tsubst_flags_t complain)
> >       }
> >
> >         /* Now compare the field subobjects.  */
> > -      for (tree field = next_initializable_field (TYPE_FIELDS (ctype));
> > +      for (tree field = next_aggregate_field (TYPE_FIELDS (ctype));
> >          field;
> > -        field = next_initializable_field (DECL_CHAIN (field)))
> > +        field = next_aggregate_field (DECL_CHAIN (field)))
> >       {
> >         if (DECL_VIRTUAL_P (field) || DECL_FIELD_IS_BASE (field))
> >           /* We ignore the vptr, and we already handled bases.  */
> > @@ -1542,7 +1542,7 @@ build_comparison_op (tree fndecl, bool defining, tsubst_flags_t complain)
> >             continue;
> >           }
> >         else if (ANON_UNION_TYPE_P (expr_type)
> > -                && next_initializable_field (TYPE_FIELDS (expr_type)))
> > +                && next_aggregate_field (TYPE_FIELDS (expr_type)))
> >           {
> >             if (complain & tf_error)
> >               inform (field_loc, "cannot default compare "
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index ac002907a41..86e7212daa5 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -29578,7 +29578,7 @@ maybe_aggr_guide (tree tmpl, tree init, vec<tree,va_gc> *args)
> >          len;
> >          --len, field = DECL_CHAIN (field))
> >       {
> > -       field = next_initializable_field (field);
> > +       field = next_aggregate_field (field);
> >         if (!field)
> >           return NULL_TREE;
> >         tree ftype = finish_decltype_type (field, true, complain);
> > diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc
> > index ed0d0d22950..aba695116c5 100644
> > --- a/gcc/cp/tree.cc
> > +++ b/gcc/cp/tree.cc
> > @@ -4852,8 +4852,8 @@ structural_type_p (tree t, bool explain)
> >       explain_non_literal_class (t);
> >         return false;
> >       }
> > -  for (tree m = next_initializable_field (TYPE_FIELDS (t)); m;
> > -       m = next_initializable_field (DECL_CHAIN (m)))
> > +  for (tree m = next_aggregate_field (TYPE_FIELDS (t)); m;
> > +       m = next_aggregate_field (DECL_CHAIN (m)))
> >       {
> >         if (TREE_PRIVATE (m) || TREE_PROTECTED (m))
> >       {
> > diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
> > index 63d95c1529a..1d92310edd0 100644
> > --- a/gcc/cp/typeck2.cc
> > +++ b/gcc/cp/typeck2.cc
> > @@ -606,7 +606,7 @@ split_nonconstant_init_1 (tree dest, tree init, bool last,
> >                                 : TYPE_FIELDS (type));
> >                    ; prev = DECL_CHAIN (prev))
> >                 {
> > -                 prev = next_initializable_field (prev);
> > +                 prev = next_aggregate_field (prev);
> >                   if (prev == field_index)
> >                     break;
> >                   tree ptype = TREE_TYPE (prev);
> > @@ -1304,7 +1304,7 @@ digest_init_r (tree type, tree init, int nested, int flags,
> >           the first element of d, which is the B base subobject.  The base
> >           of type B is copy-initialized from the D temporary, causing
> >           object slicing.  */
> > -       tree field = next_initializable_field (TYPE_FIELDS (type));
> > +       tree field = next_aggregate_field (TYPE_FIELDS (type));
> >         if (field && DECL_FIELD_IS_BASE (field))
> >           {
> >             if (warning_at (loc, 0, "initializing a base class of type %qT "
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C
> > new file mode 100644
> > index 00000000000..b3147d9db50
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7.C
> > @@ -0,0 +1,17 @@
> > +// PR c++/105491
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct V {
> > +  int m = 0;
> > +};
> > +struct S : V {
> > +  constexpr S(int) : b() { }
> > +  bool b;
> > +};
> > +struct W {
> > +  constexpr W() : s(0) { }
> > +  union {
> > +    S s;
> > +  };
> > +};
> > +constexpr W w;
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C
> > new file mode 100644
> > index 00000000000..b676e7d1748
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-union7a.C
> > @@ -0,0 +1,15 @@
> > +// PR c++/105491
> > +// { dg-do compile { target c++11 } }
> > +
> > +struct V {
> > +  int m = 0;
> > +};
> > +struct S : V {
> > +  constexpr S(int) : b() { }
> > +  bool b;
> > +};
> > +union W {
> > +  constexpr W() : s(0) { }
> > +  S s;
> > +};
> > +constexpr W w;
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/constinit17.C b/gcc/testsuite/g++.dg/cpp2a/constinit17.C
> > new file mode 100644
> > index 00000000000..6431654ac85
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/constinit17.C
> > @@ -0,0 +1,24 @@
> > +// PR c++/105491
> > +// { dg-do compile { target c++11 } }
> > +
> > +class Message {
> > +  virtual int GetMetadata();
> > +};
> > +class ProtobufCFileOptions : Message {
> > +public:
> > +  constexpr ProtobufCFileOptions(int);
> > +  bool no_generate_;
> > +  bool const_strings_;
> > +  bool use_oneof_field_name_;
> > +  bool gen_pack_helpers_;
> > +  bool gen_init_helpers_;
> > +};
> > +constexpr ProtobufCFileOptions::ProtobufCFileOptions(int)
> > +    : no_generate_(), const_strings_(), use_oneof_field_name_(),
> > +      gen_pack_helpers_(), gen_init_helpers_() {}
> > +struct ProtobufCFileOptionsDefaultTypeInternal {
> > +  constexpr ProtobufCFileOptionsDefaultTypeInternal() : _instance({}) {}
> > +  union {
> > +    ProtobufCFileOptions _instance;
> > +  };
> > +} __constinit _ProtobufCFileOptions_default_instance_;
>