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

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,
hence the function must verify that all fields of S are initialized.
And before C++17 it doesn't expect to see base class fields (since
next_initializable_field skips over them), so the presence thereof
causes r_c_e_p to return false.

The reason r10-7313-gb599bf9d6d1e18 causes this is because in that
commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether
we're in middle of activating a union member.  This ends up affecting
clear_no_implicit_zero, which recurses into sub-aggregate initializers
only if the outer initializer has CONSTRUCTOR_NO_CLEARING set.  After
that commit, the outer union initializer above no longer has the flag
set at this point and so clear_no_implicit_zero no longer recurses into
the marked inner initializer.

But arguably r_c_e_p should be able to accept the marked initializer
regardless of whether CONSTRUCTOR_NO_CLEARING is set.  The primary bug
therefore seems to be that r_c_e_p relies on next_initializable_field
which skips over base class fields in C++11/14.  To fix this, this patch
introduces a new helper function next_subobject_field which is like
next_initializable_field except that it never skips base class fields,
and makes r_c_e_p use it.  This patch then renames next_initializable_field
to next_aggregate_field (and makes it skip over vptr fields again).

	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
	again.
	(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:

	* g++.dg/cpp0x/constexpr-union7.C: New test.
	* g++.dg/cpp0x/constexpr-union7a.C: New test.
	* g++.dg/cpp2a/constinit17.C: New test.

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;
     }

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;
 	    }

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:

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));

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);

gcc/cp/decl.cc

@@ -6384,20 +6384,36 @@ static tree reshape_init_r (tree, reshape_iter *, tree, tsubst_flags_t);
 
 /* 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.  */
+   initialized as if for an aggregate class.  If there are no more such fields,
+   the return value will be NULL.  */
 
 tree
-next_initializable_field (tree field)
+next_aggregate_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.  */
+		 /* In C++17, aggregates can have bases.  */
+		 && !(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 corresponds
+   to a subobject.  If there are no more such fields, the return value will be
+   NULL.  */
+
+tree
+next_subobject_field (tree field)
+{
+  while (field
+	 && (TREE_CODE (field) != FIELD_DECL
+	     || DECL_UNNAMED_BIT_FIELD (field)
+	     || (DECL_ARTIFICIAL (field)
+		 && !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)
     {
@@ -6762,10 +6778,10 @@ 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

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);

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));

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 "

gcc/cp/pt.cc

@@ -29586,7 +29586,7 @@ 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);

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))
 	{

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 "

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;

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;

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_;