[PATCH] c++, v4: implement C++29 P2287R6 - Designated-initializers for Base Classes [PR125989]

2026-08-05  Jakub Jelinek  <[email protected]>
	    Jason Merrill  <[email protected]>

	PR c++/125989
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_designated_initializers to 202606 instead of 201707 for
	C++29.
gcc/cp/
	* parser.cc: Implement C++29 P2287R6 - Designated-initializers for
	Base Classes.
	(cp_parser_initializer_list): For C++29, allow non-designated clauses
	followed by designated ones, provided designators are identifiers.
	For C++20-26, allow that as a pedwarned about extension but use the
	C++20 wording in that case.
	* decl.cc (reshape_init_class): Diagnose non-designated clause not
	appertaining to base class followed by designated clause for C++ >= 20.
	For C++29, if get_class_binding for designator is unsuccessful, use
	lookup_member afterwards and handle the case where the lookup succeeds
	for some member of a base class through recursive reshape_init_class.
gcc/testsuite/
	* g++.dg/cpp/embed-14.C: Expect different diagnostics for C++29 and
	sometimes for C++20-C++26 too.
	* g++.dg/cpp2a/desig2.C: Likewise.
	* g++.dg/cpp2a/desig13.C: Likewise.
	* g++.dg/cpp2a/desig20.C: Likewise.
	* g++.dg/ext/desig4.C: Likewise.
	* g++.dg/parse/pr43765.C: Likewise.
	* g++.dg/cpp29/feat-cxx29.C: Expect different value of
	__cpp_designated_initializers for C++29.
	* g++.dg/cpp29/desig1.C: New test.
	* g++.dg/cpp29/desig2.C: New test.
	* g++.dg/cpp29/desig3.C: New test.
	* g++.dg/cpp29/desig4.C: New test.
	* g++.dg/cpp29/desig5.C: New test.
	* g++.dg/cpp29/desig6.C: New test.
	* g++.dg/cpp29/desig7.C: New test.
	* g++.dg/cpp29/desig8.C: New test.
	* g++.dg/cpp29/desig9.C: New test.
	* g++.dg/cpp29/desig10.C: New test.
	* g++.dg/cpp29/desig11.C: New test.
	* g++.dg/cpp29/desig12.C: New test.
	* g++.dg/cpp29/desig13.C: New test.
	* g++.dg/cpp29/desig14.C: New test.
	* g++.dg/cpp29/desig15.C: New test.
	* g++.dg/cpp29/desig16.C: New test.

--- gcc/c-family/c-cppbuiltin.cc.jj	2026-07-27 12:45:24.349871210 +0200
+++ gcc/c-family/c-cppbuiltin.cc	2026-08-05 12:33:10.560785823 +0200
@@ -1072,7 +1072,8 @@ c_cpp_builtins (cpp_reader *pfile)
 	  /* Set feature test macros for C++20.  */
 	  cpp_define (pfile, "__cpp_init_captures=201803L");
 	  cpp_define (pfile, "__cpp_generic_lambdas=201707L");
-	  cpp_define (pfile, "__cpp_designated_initializers=201707L");
+	  if (cxx_dialect <= cxx26)
+	    cpp_define (pfile, "__cpp_designated_initializers=201707L");
 	  if (cxx_dialect <= cxx20)
 	    cpp_define (pfile, "__cpp_constexpr=202002L");
 	  cpp_define (pfile, "__cpp_constexpr_in_decltype=201711L");
@@ -1129,6 +1130,7 @@ c_cpp_builtins (cpp_reader *pfile)
 	{
 	  /* Set feature test macros for C++29.  */
 	  cpp_define (pfile, "__cpp_pp_embed=202606L");
+	  cpp_define (pfile, "__cpp_designated_initializers=202606L");
 	}
       if (flag_concepts && cxx_dialect > cxx14)
 	cpp_define (pfile, "__cpp_concepts=202002L");
--- gcc/cp/parser.cc.jj	2026-08-04 10:27:31.735878042 +0200
+++ gcc/cp/parser.cc	2026-08-05 12:33:10.566785749 +0200
@@ -29408,13 +29408,35 @@ cp_parser_initializer_list (cp_parser* p
 	}
       else if (cxx_dialect >= cxx20
 	       && first_designator != error_mark_node
-	       && (!first_designator != !designator))
+	       && (!first_designator != !designator)
+	       && (cxx_dialect < cxx29
+		   || first_designator
+		   || TREE_CODE (designator) != IDENTIFIER_NODE))
 	{
-	  error_at (loc, "either all initializer clauses should be designated "
-			 "or none of them should be");
-	  first_designator = error_mark_node;
+	  if (cxx_dialect < cxx29
+	      && !first_designator
+	      && TREE_CODE (designator) == IDENTIFIER_NODE)
+	    {
+	      pedwarn (loc, OPT_Wc__29_extensions,
+		       "either all initializer clauses should be "
+		       "designated or none of them should be");
+	      first_designator = designator;
+	    }
+	  else
+	    {
+	      if (cxx_dialect < cxx29
+		  || TREE_CODE (first_designator
+				? first_designator
+				: designator) != IDENTIFIER_NODE)
+		error_at (loc, "either all initializer clauses should be "
+			       "designated or none of them should be");
+	      else
+		error_at (loc, "designated initializer clause should not "
+			       "be followed by non-designated");
+	      first_designator = error_mark_node;
+	    }
 	}
-      else if (cxx_dialect < cxx20 && !first_designator)
+      else if (!first_designator)
 	first_designator = designator;
 
       /* Parse the initializer.  */
--- gcc/cp/decl.cc.jj	2026-07-27 12:45:24.350871197 +0200
+++ gcc/cp/decl.cc	2026-08-05 12:33:50.858291920 +0200
@@ -7829,8 +7829,25 @@ reshape_init_class (tree type, reshape_i
       return new_init;
     }
 
+  /* For C++29 designated initializers we do modify d->cur->index in place
+     to cache name lookup results.  Make sure to undo it before returning.  */
+  struct designator_undo {
+    constructor_elt *start, *end;
+    void undo ()
+    {
+      while (start != end)
+	{
+	  start->index = DECL_NAME (start->index);
+	  ++start;
+	}
+      start = end = nullptr;
+    }
+    ~designator_undo () { undo (); }
+  } desig_undo = { nullptr, nullptr };
+
   /* For C++20 CTAD, handle pack expansions in the base list.  */
   tree last_was_pack_expansion = NULL_TREE;
+  bool first_desig = true;
 
   /* Loop through the initializable fields, gathering initializers.  */
   while (d->cur != d->end)
@@ -7839,6 +7856,7 @@ reshape_init_class (tree type, reshape_i
       constructor_elt *old_cur = d->cur;
       unsigned old_raw_idx = d->raw_idx;
       bool direct_desig = false;
+      bool subclass = false;
 
       /* Handle C++20 designated initializers.  */
       if (d->cur->index)
@@ -7848,22 +7866,37 @@ reshape_init_class (tree type, reshape_i
 
 	  if (TREE_CODE (d->cur->index) == FIELD_DECL)
 	    {
-	      /* We already reshaped this; we should have returned early from
-		 reshape_init.  */
-	      gcc_checking_assert (false);
-	      if (field != d->cur->index)
-		{
-		  if (tree id = DECL_NAME (d->cur->index))
-		    gcc_checking_assert (d->cur->index
-					 == get_class_binding (type, id));
-		  field = d->cur->index;
-		}
+	      CONSTRUCTOR_IS_DESIGNATED_INIT (new_init) = true;
+	      direct_desig = true;
+	      field = d->cur->index;
 	    }
 	  else if (TREE_CODE (d->cur->index) == IDENTIFIER_NODE)
 	    {
+	      if (first_desig && cxx_dialect >= cxx20)
+		{
+		  if (CONSTRUCTOR_NELTS (new_init))
+		    {
+		      constructor_elt *last
+			= &CONSTRUCTOR_ELTS (new_init)->last ();
+		      if (last->index == NULL_TREE
+			  || TREE_CODE (last->index) != FIELD_DECL
+			  || !DECL_FIELD_IS_BASE (last->index))
+			{
+			  if (complain & tf_error)
+			    error ("last non-designated initializer clause "
+				   "does not appertain to a base class "
+				   "subobject");
+			  return error_mark_node;
+			}
+		    }
+		  first_desig = false;
+		}
 	      CONSTRUCTOR_IS_DESIGNATED_INIT (new_init) = true;
 	      field = get_class_binding (type, d->cur->index);
 	      direct_desig = true;
+	      if (!field && cxx_dialect >= cxx29)
+		field = lookup_member (type, d->cur->index, /*protect=*/2,
+				       /*want_type=*/false, complain);
 	    }
 	  else
 	    {
@@ -7913,6 +7946,62 @@ reshape_init_class (tree type, reshape_i
 		  ictx = cctx;
 		}
 
+	      /* In C++29 a designator can name a member of a base; in that
+		 case, go through the designators and replace ids with _DECLs
+		 to record the lookup for the most-derived class.  */
+	      if (cxx_dialect >= cxx29)
+		{
+		  tree ibinfo = lookup_base (type, ictx, ba_unique, NULL,
+					     complain);
+		  if (!ibinfo)
+		    /* The designator names a field outside this base class,
+		       so we're done.  */
+		    break;
+		  else if (ibinfo != error_mark_node)
+		    {
+		      while (BINFO_INHERITANCE_CHAIN (ibinfo) != binfo)
+			ibinfo = BINFO_INHERITANCE_CHAIN (ibinfo);
+		      ictx = TREE_TYPE (ibinfo);
+
+		      desig_undo.undo ();
+
+		      if (d->cur->index != field)
+			{
+			  d->cur->index = field;
+			  desig_undo.start = d->cur;
+			}
+		      constructor_elt *e = d->cur + 1;
+		      for (; e != d->end; ++e)
+			{
+			  if (e->index == NULL_TREE
+			      || e->index == error_mark_node)
+			    break;
+			  if (desig_undo.start)
+			    {
+			      gcc_assert (TREE_CODE (e->index)
+					  == IDENTIFIER_NODE);
+			      field = lookup_member (type, e->index,
+						     /*protect=*/2,
+						     /*want_type=*/false,
+						     tf_none);
+			      if (!field || TREE_CODE (field) != FIELD_DECL)
+				break;
+			    }
+			  else
+			    {
+			      gcc_assert (TREE_CODE (e->index) == FIELD_DECL);
+			      field = e->index;
+			    }
+
+			  if (desig_undo.start)
+			    e->index = field;
+			}
+		      if (desig_undo.start)
+			desig_undo.end = e;
+		      goto found;
+		    }
+		}
+
 	      /* Not found, e.g. FIELD is a member of a base class.  */
 	      if (complain & tf_error)
 		error ("%qD is not a direct member of %qT", field, type);
@@ -7927,6 +8016,7 @@ reshape_init_class (tree type, reshape_i
 	      gcc_assert (aafield);
 	      field = aafield;
 	      direct_desig = false;
+	      subclass = true;
 	    }
 	}
 
@@ -7958,6 +8048,18 @@ reshape_init_class (tree type, reshape_i
 					    d->cur->value, complain);
 	  d->cur++;
 	}
+      else if (subclass)
+	{
+	  if (complain & tf_warning)
+	    warning (OPT_Wmissing_braces,
+		     "missing braces around initializer for %qT",
+		     TREE_TYPE (field));
+	  field_init = reshape_init_class (TREE_TYPE (field), d,
+					   /*first_initializer_p=*/NULL_TREE,
+					   complain);
+	  if (TREE_CODE (field_init) == CONSTRUCTOR)
+	    CONSTRUCTOR_BRACES_ELIDED_P (field_init) = true;
+	}
       else
 	field_init = reshape_init_r (TREE_TYPE (field), d,
 				     /*first_initializer_p=*/NULL_TREE,
--- gcc/testsuite/g++.dg/cpp/embed-14.C.jj	2026-07-27 12:45:24.363871030 +0200
+++ gcc/testsuite/g++.dg/cpp/embed-14.C	2026-08-05 12:33:10.571623985 +0200
@@ -3,8 +3,8 @@
 
 struct S { int a; long b; unsigned char c[63]; int d; };
 S s = {
-#embed __FILE__ limit (64) prefix (.a = 1, .b = ) suffix (, .d = 2)	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } }
-};
+#embed __FILE__ limit (64) prefix (.a = 1, .b = ) suffix (, .d = 2)	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } }
+};									// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
 const unsigned char t[66] = {
 #embed __FILE__ limit (64) prefix ([0] = 1, [1] =) suffix (, [65] = 2)	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } }
 };
--- gcc/testsuite/g++.dg/cpp2a/desig2.C.jj	2026-07-27 12:45:24.363871030 +0200
+++ gcc/testsuite/g++.dg/cpp2a/desig2.C	2026-08-05 12:33:10.571793821 +0200
@@ -5,8 +5,10 @@ struct S { int a, b, c; };
 
 S a = { 1, 2, 3 };
 S b = { .a = 1, .b = 2, .c = 3 };
-S c = { 1, .b = 2, .c = 3 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } }
-S d = { .a = 1, 2, 3 };		// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } }
+S c = { 1, .b = 2, .c = 3 };	// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } }
+				// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" "" { target c++20 } .-1 }
+S d = { .a = 1, 2, 3 };		// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } }
+				// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
 S e = { .b = 1, .b = 2 };	// { dg-error "designator used multiple times in the same initializer list" }
 
 #if __cplusplus > 201103L
--- gcc/testsuite/g++.dg/cpp2a/desig13.C.jj	2026-07-27 12:45:24.363871030 +0200
+++ gcc/testsuite/g++.dg/cpp2a/desig13.C	2026-08-05 12:33:10.571953478 +0200
@@ -11,6 +11,7 @@ void
 baz ()
 {
   foo ({.d = 5, 6, .b = 2, 3});	// { dg-error "designator order for field 'S::b' does not match declaration order in 'S'" }
-				// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } .-1 }
+				// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } .-1 }
+				// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-2 }
   bar ({.b = 1, .a = 2});	// { dg-error "designator order for field 'T::a' does not match declaration order in 'T'" }
 }
--- gcc/testsuite/g++.dg/cpp2a/desig20.C.jj	2026-07-27 12:45:24.363871030 +0200
+++ gcc/testsuite/g++.dg/cpp2a/desig20.C	2026-08-05 12:33:10.572080321 +0200
@@ -16,5 +16,5 @@ struct B : A {
 int main()
 {
   [[maybe_unused]] B b =
-  { .a = 10, .d = 42 };		// { dg-error "not a direct member" }
+  { .a = 10, .d = 42 };		// { dg-error "not a direct member" "" { target c++26_down } }
 }
--- gcc/testsuite/g++.dg/ext/desig4.C.jj	2026-07-27 12:45:24.364871017 +0200
+++ gcc/testsuite/g++.dg/ext/desig4.C	2026-08-05 12:33:10.572222207 +0200
@@ -5,10 +5,11 @@ char g[] = { [7] = "abcd" };	     // { d
 int a = { .foo = 6 };		     // { dg-error "designator" }
 int b = { [0] = 1 };		     // { dg-error "12:designator .0." }
 _Complex float c = { .foo = 0,  1 }; // { dg-error "designator" }
-				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++2a } .-1 }
+				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } .-1 }
+				     // { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-2 }
 _Complex float d = { [0] = 0,  1 };  // { dg-error "23:designator .0." }
-				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++2a } .-1 }
+				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } .-1 }
 _Complex float e = { 0, .foo = 1 };  // { dg-error "designator" }
-				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++2a } .-1 }
+				     // { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } .-1 }
 _Complex float f = { 0, [0] = 1 };   // { dg-error "26:designator .0." }
-				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++2a } .-1 }
+				     // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++20 } .-1 }
--- gcc/testsuite/g++.dg/parse/pr43765.C.jj	2026-07-27 12:45:24.364871017 +0200
+++ gcc/testsuite/g++.dg/parse/pr43765.C	2026-08-05 12:33:10.572357776 +0200
@@ -10,8 +10,9 @@ const char *temp[] = {"607", "612", 0};
 
 SomeType vals[] =
     {
-        { 0, values : temp, },	 // { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++2a } }
+	{ 0, values : temp, },	 // { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target { c++20 && c++26_down } } }
         0
     };
 // (note the error below is on the wrong line)
-// { dg-error "initialization of flexible array member in a nested context" "" { target *-*-* } .-2 }
+// { dg-error "initialization of flexible array member in a nested context" "" { target c++17_down } .-2 }
+// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" "" { target c++20 } .-3 }
--- gcc/testsuite/g++.dg/cpp29/feat-cxx29.C.jj	2026-07-27 12:45:24.363871030 +0200
+++ gcc/testsuite/g++.dg/cpp29/feat-cxx29.C	2026-08-05 12:33:10.572895114 +0200
@@ -469,8 +469,8 @@
 
 #ifndef __cpp_designated_initializers
 #  error "__cpp_designated_initializers"
-#elif __cpp_designated_initializers != 201707
-#  error "__cpp_designated_initializers != 201707"
+#elif __cpp_designated_initializers != 202606
+#  error "__cpp_designated_initializers != 202606"
 #endif
 
 #ifndef __cpp_constexpr_in_decltype
--- gcc/testsuite/g++.dg/cpp29/desig1.C.jj	2026-08-05 12:33:10.573049148 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig1.C	2026-08-05 12:33:10.573049148 +0200
@@ -0,0 +1,12 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct A { int a; };
+struct B : A { int b; };
+auto b1 = B { { 1 }, 2 };
+auto b2 = B { 1, 2 };
+auto b3 = B { .a = 1, .b = 2 };		// { dg-error "'B' has no non-static data member named 'a'" "" { target c++26_down } }
+auto b4 = B { { .a = 1 }, .b = 2 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto b5 = B { .a { 1 }, .b { 2 } };	// { dg-error "'B' has no non-static data member named 'a'" "" { target c++26_down } }
+auto b6 = B { .b = 2, .a = 1 };		// { dg-error "designator order for field 'B::A' does not match declaration order in 'B'" "" { target c++29 } }
+					// { dg-error "'B' has no non-static data member named 'a'" "" { target c++26_down } .-1 }
--- gcc/testsuite/g++.dg/cpp29/desig2.C.jj	2026-08-05 12:33:10.573137909 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig2.C	2026-08-05 12:33:10.573137909 +0200
@@ -0,0 +1,9 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct A { A (const char *); int a; };
+struct B : A { int b, c; };
+auto b1 = B { "hello", .b = 3, .c = 4 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto b2 = B { { "hello" }, .b = 3, .c = 4 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto b3 = B { "nope", 3, .c = 4 };		// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+						// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }
--- gcc/testsuite/g++.dg/cpp29/desig3.C.jj	2026-08-05 12:33:10.573225565 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig3.C	2026-08-05 12:33:10.573225565 +0200
@@ -0,0 +1,41 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int a; };
+struct B : A { int b; };
+constexpr auto b1 = B { { 1 }, 2 };
+constexpr auto b2 = B { 3, 4 };
+constexpr auto b3 = B { .a = 5, .b = 6 };
+constexpr auto b4 = B { { .a = 7 }, .b = 8 };
+constexpr auto b5 = B { .a { 9 }, .b { 10 } };
+static_assert (b1.a == 1 && b1.b == 2);
+static_assert (b2.a == 3 && b2.b == 4);
+static_assert (b3.a == 5 && b3.b == 6);
+static_assert (b4.a == 7 && b4.b == 8);
+static_assert (b5.a == 9 && b5.b == 10);
+
+struct C { constexpr C (const char *x) : a (0) { while (*x) a += *x++; } int a; };
+struct D : C { int b, c; };
+constexpr auto d1 = D { "abc", .b = 3, .c = 4 };
+constexpr auto d2 = D { { "de" }, .b = 5, .c = 6 };
+static_assert (d1.a == 'a' + 'b' + 'c' && d1.b == 3 && d1.c == 4);
+static_assert (d2.a == 'd' + 'e' && d2.b == 5 && d2.c == 6);
+
+struct E { int x; };
+struct F : E { int x; };
+constexpr auto f1 = F { .x = 1 };
+constexpr auto f2 = F { { .x = 2 }, .x = 3 };
+constexpr auto f3 = F { E { 4 }, .x = 5 };
+static_assert (f1.E::x == 0 && f1.F::x == 1);
+static_assert (f2.E::x == 2 && f2.F::x == 3);
+static_assert (f3.E::x == 4 && f3.F::x == 5);
+
+struct G { int g; };
+struct H { int h; };
+struct I : G, H { int i; };
+constexpr auto i1 = I { { .g = 1 }, { .h = 2 }, .i = 3 };
+constexpr auto i2 = I { { .g = 4 }, .h = 5, .i = 6 };
+constexpr auto i3 = I { { .g = 7 }, H { 8 }, .i = 9 };
+static_assert (i1.g == 1 && i1.h == 2 && i1.i == 3);
+static_assert (i2.g == 4 && i2.h == 5 && i2.i == 6);
+static_assert (i3.g == 7 && i3.h == 8 && i3.i == 9);
--- gcc/testsuite/g++.dg/cpp29/desig4.C.jj	2026-08-05 12:33:10.573338790 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig4.C	2026-08-05 12:33:10.573338790 +0200
@@ -0,0 +1,13 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct F { int f; };
+struct G { int g; };
+struct H : F, G { int h; };
+auto h1 = H { { .f = 1 }, { .g = 2 }, .h = 3 };		// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto h2 = H { { .f = 1 }, .g = 2, .h = 3 };		// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+							// { dg-error "'H' has no non-static data member named 'g'" "" { target c++26_down } .-1 }
+auto h3 = H { { .f = 1 }, G { 2 }, .h = 3 };		// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto h4 = H { { .f = 1 }, { .g = 2 }, .g = 3, .h = 4 };	// { dg-error "designator order for field 'H::G' does not match declaration order in 'H'" "" { target c++29 } }
+							// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }
+							// { dg-error "'H' has no non-static data member named 'g'" "" { target c++26_down } .-2 }
--- gcc/testsuite/g++.dg/cpp29/desig5.C.jj	2026-08-05 12:33:10.573429982 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig5.C	2026-08-05 12:33:10.573429982 +0200
@@ -0,0 +1,25 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct A { int a; };
+struct B : A { int b; };
+struct C : A { C (); int c; };
+struct D : C { int d; };
+
+auto a = A { .a = 1 };
+auto b = B { .a = 1, .b = 2 };	// { dg-error "'B' has no non-static data member named 'a'" "" { target c++26_down } }
+auto c = C { .c = 1 };		// { dg-error "designated initializers cannot be used with a non-aggregate type 'C'" }
+				// { dg-error "no matching function for call to" "" { target *-*-* } .-1 }
+auto d = D { .a = 1 };		// { dg-error "designated initializers cannot be used with a non-aggregate type 'C'" "" { target c++29 } }
+				// { dg-error "'D' has no non-static data member named 'a'" "" { target c++26_down } .-1 }
+
+struct E { int x; };
+struct F : E { int x; };
+constexpr auto f = F { .x = 1 };
+static_assert (f.E::x == 0 && f.F::x == 1);
+
+struct G { int x; };
+struct H { int x; };
+struct I : G, H { };
+auto i = I { .x = 1 };		// { dg-error "request for member 'x' is ambiguous" "" { target c++29 } }
+				// { dg-error "'I' has no non-static data member named 'x'" "" { target c++26_down } .-1 }
--- gcc/testsuite/g++.dg/cpp29/desig6.C.jj	2026-08-05 12:33:10.573513578 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig6.C	2026-08-05 12:33:10.573513578 +0200
@@ -0,0 +1,16 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int a1, a2; };
+struct B : A { int b; };
+struct C : A { int a1; };
+A v0 = A { 1, .a2 = 2 };			// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+constexpr B v1 = B { .a1 = 1, .b = 2 };		// the explicitly initialized elements are [A, B::b]
+static_assert (v1.a1 == 1 && v1.a2 == 0 && v1.b == 2);
+constexpr B v2 = B { .a1 = 1, .a2 = 2, .b = 3 };// the explicitly initialized elements are [A, B::b]
+static_assert (v2.a1 == 1 && v2.a2 == 2 && v2.b == 3);
+constexpr B v3 = B { A { 1, 2 }, .b = 3 };	// the explicitly initialized elements are [A, B::b]
+static_assert (v3.a1 == 1 && v3.a2 == 2 && v3.b == 3);
+B v4 = B { A { }, .a2 = 1, .b = 3 };		// { dg-error "designator order for field 'B::A' does not match declaration order in 'B'" }
+constexpr C v5 = C { .a1 = 4 };			// the explicitly initialized elements are [C::a1]
+static_assert (v5.A::a1 == 0 && v5.a2 == 0 && v5.C::a1 == 4);
--- gcc/testsuite/g++.dg/cpp29/desig7.C.jj	2026-08-05 12:33:10.573602181 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig7.C	2026-08-05 12:33:10.573602181 +0200
@@ -0,0 +1,10 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int a; };
+struct B : A { int b; };
+struct C : B { int c; };
+constexpr B x = B { .a = 1 };
+static_assert (x.a == 1 && x.b == 0);
+constexpr C y = C { .a = 2, .b = 3, .c = 4 };
+static_assert (y.a == 2 && y.b == 3 && y.c == 4);
--- gcc/testsuite/g++.dg/cpp29/desig8.C.jj	2026-08-05 12:33:10.573681626 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig8.C	2026-08-05 12:33:10.573681626 +0200
@@ -0,0 +1,19 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int x; int y; int z; };
+A a { .y = 2, .x = 1 };			// { dg-error "designator order for field 'A::x' does not match declaration order in 'A'" }
+constexpr A b { .x = 1, .z = 2 };
+static_assert (b.x == 1 && b.y == 0 && b.z == 2);
+struct B : A { int q; };
+constexpr B e { .x = 1, .q = 3 };
+static_assert (e.x == 1 && e.y == 0 && e.z == 0 && e.q == 3);
+B f { .q = 3, .x = 1 };			// { dg-error "designator order for field 'B::A' does not match declaration order in 'B'" }
+struct C { int p; int x; };
+struct D : A, C { };
+constexpr D g { .y = 1, .p = 2 };
+static_assert (g.A::x == 0 && g.y == 1 && g.z == 0 && g.p == 2 && g.C::x == 0);
+D h { .x = 2 };				// { dg-error "request for member 'x' is ambiguous" }
+struct NonAggr { int na; NonAggr (int); };
+struct E : NonAggr { int e; };
+E i { .na = 1, .e = 2 };		// { dg-error "designated initializers cannot be used with a non-aggregate type 'NonAggr'" }
--- gcc/testsuite/g++.dg/cpp29/desig9.C.jj	2026-08-05 12:33:10.573761610 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig9.C	2026-08-05 12:33:10.573761610 +0200
@@ -0,0 +1,12 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct A { int a; };
+struct B : A { int b; };
+void foo (A);			// { dg-message "candidate 1: 'void foo\\\(A\\\)'" "" { target c++29 } }
+void foo (B);			// { dg-message "candidate 2: 'void foo\\\(B\\\)'" "" { target c++29 } }
+void
+bar ()
+{
+  foo ({ .a = 1 });		// { dg-error "call of overloaded 'foo\\\(<brace-enclosed initializer list>\\\)' is ambiguous" "" { target c++29 } }
+}				// { dg-message "there are 2 candidates" "" { target c++29 } .-1 }
--- gcc/testsuite/g++.dg/cpp29/desig10.C.jj	2026-08-05 12:33:10.573846748 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig10.C	2026-08-05 12:33:10.573846748 +0200
@@ -0,0 +1,43 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int a, b, c; };
+struct B { int d, e; };
+struct C : A { int f, g; };
+struct D : B { int h, i; };
+struct E : C, D { int j; };
+constexpr auto e1 = E { .a = 1, .c = 2, .f = 3, .d = 4, .i = 5, .j = 6 };
+static_assert (e1.a == 1 && e1.b == 0 && e1.c == 2 && e1.f == 3 && e1.g == 0
+	       && e1.d == 4 && e1.e == 0 && e1.h == 0 && e1.i == 5 && e1.j == 6);
+auto e2 = E { 1, 2, 3, 4, 5, .j = 6 };	// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+constexpr auto e3 = E { { .a = 1, .b = 2 }, .j = 3 };
+static_assert (e3.a == 1 && e3.b == 2 && e3.c == 0 && e3.f == 0 && e3.g == 0
+	       && e3.d == 0 && e3.e == 0 && e3.h == 0 && e3.i == 0 && e3.j == 3);
+constexpr auto e4 = E { {}, { { .e = 1 }, .i = 2 }, .j = 3 };
+static_assert (e4.a == 0 && e4.b == 0 && e4.c == 0 && e4.f == 0 && e4.g == 0
+	       && e4.d == 0 && e4.e == 1 && e4.h == 0 && e4.i == 2 && e4.j == 3);
+constexpr auto e5 = E { { { .b = 1 }, .f = 2 }, { { .d = 3 }, .h = 4 }, .j = 5 };
+static_assert (e5.a == 0 && e5.b == 1 && e5.c == 0 && e5.f == 2 && e5.g == 0
+	       && e5.d == 3 && e5.e == 0 && e5.h == 4 && e5.i == 0 && e5.j == 5);
+constexpr auto e6 = E { .a = 1, .b = 2, .c = 3, .f = 4, .g = 5, .d = 6,
+			.e = 7, .h = 8, .i = 9, .j = 10 };
+static_assert (e6.a == 1 && e6.b == 2 && e6.c == 3 && e6.f == 4 && e6.g == 5
+	       && e6.d == 6 && e6.e == 7 && e6.h == 8 && e6.i == 9 && e6.j == 10);
+auto e7 = E { .a = 1, .b = 2, .c = 3, .d = 4, .e = 5, .f = 6, .g = 7,
+	      .h = 8, .i = 9, .j = 10 };// { dg-error "designator order for field 'E::C' does not match declaration order in 'E'" }
+auto e8 = E { {}, {}, .a = 1, .j = 2 };	// { dg-error "designator order for field 'E::C' does not match declaration order in 'E'" }
+auto e9 = E { {}, .f = 1 };		// { dg-error "designator order for field 'E::C' does not match declaration order in 'E'" }
+auto e10 = E { {}, {}, .e = 1 };	// { dg-error "designator order for field 'E::D' does not match declaration order in 'E'" }
+auto e11 = E { {}, {}, .h = 1 };	// { dg-error "designator order for field 'E::D' does not match declaration order in 'E'" }
+auto e12 = E { .a = 0, .b = 0, .c = 0, .d = 0, .e = 0, .f = 0, .g = 0, .h = 0, .i = 0, .j = 0,
+	       .a = 0, .b = 0, .c = 0, .d = 0, .e = 0, .f = 0, .g = 0, .h = 0, .i = 0, .j = 0 };
+// { dg-error "'.a' designator used multiple times in the same initializer list" "" { target *-*-* } .-1 }
+// { dg-error "'.b' designator used multiple times in the same initializer list" "" { target *-*-* } .-2 }
+// { dg-error "'.c' designator used multiple times in the same initializer list" "" { target *-*-* } .-3 }
+// { dg-error "'.d' designator used multiple times in the same initializer list" "" { target *-*-* } .-4 }
+// { dg-error "'.e' designator used multiple times in the same initializer list" "" { target *-*-* } .-5 }
+// { dg-error "'.f' designator used multiple times in the same initializer list" "" { target *-*-* } .-6 }
+// { dg-error "'.g' designator used multiple times in the same initializer list" "" { target *-*-* } .-7 }
+// { dg-error "'.h' designator used multiple times in the same initializer list" "" { target *-*-* } .-8 }
+// { dg-error "'.i' designator used multiple times in the same initializer list" "" { target *-*-* } .-9 }
+// { dg-error "'.j' designator used multiple times in the same initializer list" "" { target *-*-* } .-10 }
--- gcc/testsuite/g++.dg/cpp29/desig11.C.jj	2026-08-05 12:33:10.573940360 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig11.C	2026-08-05 12:33:10.573940360 +0200
@@ -0,0 +1,21 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++29 } }
+
+struct A { int a, x; };
+struct B { int b, x; };
+struct C : A, B { int c; };
+struct D : C { int x; };
+constexpr auto d = D { .c = 1, .x = 2 };
+static_assert (d.a == 0 && d.A::x == 0 && d.b == 0 && d.B::x == 0
+	       && d.c == 1 && d.D::x == 2);
+struct E { int a, x; };
+struct F : E { int x; };
+constexpr auto f = F { .a = 1, .x = 2 };
+static_assert (f.a == 1 && f.E::x == 0 && f.F::x == 2);
+struct G { int a, b; };
+struct H : G { int c; };
+struct I { int a, b, d; };
+struct J : I { int e, c; };
+constexpr int foo (H, G) { return 1; }
+constexpr int foo (J, I) { return 42; }
+static_assert (foo ({ .a = 1, .b = 2, .c = 3 }, { .d = 1 }) == 42);
--- gcc/testsuite/g++.dg/cpp29/desig12.C.jj	2026-08-05 12:33:10.574033956 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig12.C	2026-08-05 12:33:10.574033956 +0200
@@ -0,0 +1,13 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wno-c++29-extensions" }
+
+struct A { int a, b; };
+struct B { int c, d; };
+struct C : A, B { int e, f; };
+auto c1 = C { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 };
+auto c2 = C { { .a = 1, .b = 2 }, .e = 5, .f = 6 };
+auto c3 = C { {}, { .c = 3, .d = 4 }, .f = 6 };
+auto c4 = C { .e = 1, 2 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+				// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
+auto a1 = A { 1, .b = 2 };	// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
--- gcc/testsuite/g++.dg/cpp29/desig13.C.jj	2026-08-05 12:33:10.574115783 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig13.C	2026-08-05 12:33:10.574115783 +0200
@@ -0,0 +1,14 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wc++29-extensions" }
+
+struct A { int a, b; };
+struct B { int c, d; };
+struct C : A, B { int e, f; };
+auto c1 = C { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 };	// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c2 = C { { .a = 1, .b = 2 }, .e = 5, .f = 6 };			// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c3 = C { {}, { .c = 3, .d = 4 }, .f = 6 };				// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c4 = C { .e = 1, 2 };						// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+									// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
+auto a1 = A { 1, .b = 2 };						// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+									// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }
--- gcc/testsuite/g++.dg/cpp29/desig14.C.jj	2026-08-05 12:33:10.574200452 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig14.C	2026-08-05 12:57:26.171939454 +0200
@@ -0,0 +1,23 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wmissing-braces" }
+
+struct A { int a, b; };
+struct B { int c, d; };
+struct C : A, B { int e, f; };
+struct D : C { int g; };
+auto c1 = C { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 };	// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c2 = C { { .a = 1, .b = 2 }, .e = 5, .f = 6 };			// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c3 = C { {}, { .c = 3, .d = 4 }, .f = 6 };				// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c4 = C { .e = 1, 2 };						// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+									// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
+auto c5 = C { .a = 1, .b = 2, .c = 3, .d = 4, .e = 5, .f = 6 };		// { dg-error "'C' has no non-static data member named 'a'" "" { target c++26_down } }
+									// { dg-warning "missing braces around initializer for 'A'" "" { target c++29 } .-1 }
+									// { dg-warning "missing braces around initializer for 'B'" "" { target c++29 } .-2 }
+auto a1 = A { 1, .b = 2 };						// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+									// { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }
+auto d1 = D { .a = 1, .b = 2, .c = 3, .d = 4, .e = 5, .f = 6, .g = 7 };	// { dg-error "'D' has no non-static data member named 'a'" "" { target c++26_down } }
+									// { dg-warning "missing braces around initializer for 'A'" "" { target c++29 } .-1 }
+									// { dg-warning "missing braces around initializer for 'B'" "" { target c++29 } .-2 }
+									// { dg-warning "missing braces around initializer for 'C'" "" { target c++29 } .-3 }
+auto d2 = D { { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 }, .g = 7 }; // { dg-warning "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
--- gcc/testsuite/g++.dg/cpp29/desig15.C.jj	2026-08-05 12:33:10.574280785 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig15.C	2026-08-05 12:33:10.574280785 +0200
@@ -0,0 +1,15 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+// { dg-options "-Werror=c++29-extensions" }
+
+struct A { int a, b; };
+struct B { int c, d; };
+struct C : A, B { int e, f; };
+auto c1 = C { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c2 = C { { .a = 1, .b = 2 }, .e = 5, .f = 6 };			// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c3 = C { {}, { .c = 3, .d = 4 }, .f = 6 };				// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c4 = C { .e = 1, 2 };						// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+									// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
+auto a1 = A { 1, .b = 2 };						// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+									// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }
+									// { dg-message "some warnings being treated as errors" "" { target c++26_down } 0 }
--- gcc/testsuite/g++.dg/cpp29/desig16.C.jj	2026-08-05 12:33:10.574369740 +0200
+++ gcc/testsuite/g++.dg/cpp29/desig16.C	2026-08-05 12:33:10.574369740 +0200
@@ -0,0 +1,13 @@
+// C++29 P2287R6 - Designated-initializers for Base Classes
+// { dg-do compile { target c++20 } }
+
+struct A { int a, b; };
+struct B { int c, d; };
+struct C : A, B { int e, f; };
+auto c1 = C { { .a = 1, .b = 2 }, { .c = 3, .d = 4 }, .e = 5, .f = 6 };	// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c2 = C { { .a = 1, .b = 2 }, .e = 5, .f = 6 };			// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c3 = C { {}, { .c = 3, .d = 4 }, .f = 6 };				// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+auto c4 = C { .e = 1, 2 };						// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } }
+									// { dg-error "designated initializer clause should not be followed by non-designated" "" { target c++29 } .-1 }
+auto a1 = A { 1, .b = 2 };						// { dg-error "last non-designated initializer clause does not appertain to a base class subobject" }
+									// { dg-error "either all initializer clauses should be designated or none of them should be" "" { target c++26_down } .-1 }