From db29b66193407d2fd6a04a07ecbd961c8d3f7d62 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Thu, 23 Jun 2022 16:04:02 -0400
Subject: [PATCH] c++: anon union designated init [PR105925]

This testcase was failing because CONSTRUCTOR_IS_DESIGNATED_INIT wasn't
getting set on the introduced CONSTRUCTOR for the anonymous union, and
build_aggr_conv uses that flag to decide whether to pay attention to the
indexes of the CONSTRUCTOR.  So set the flag when we see a designator rather
than relying on copying it from another CONSTRUCTOR.

	PR c++/105925

gcc/cp/ChangeLog:

	* decl.cc (reshape_init_array_1): Set
	CONSTRUCTOR_IS_DESIGNATED_INIT here.
	(reshape_init_class): And here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/desig26.C: New test.
---
 gcc/cp/decl.cc                       |  3 +++
 gcc/testsuite/g++.dg/cpp2a/desig26.C | 22 ++++++++++++++++++++++
 2 files changed, 25 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/desig26.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 66d62af58f1c..42c8d0b32e99 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -6503,6 +6503,8 @@ reshape_init_array_1 (tree elt_type, tree max_index, reshape_iter *d,
       tree elt_init;
       constructor_elt *old_cur = d->cur;
 
+      if (d->cur->index)
+	CONSTRUCTOR_IS_DESIGNATED_INIT (new_init) = true;
       check_array_designated_initializer (d->cur, index);
       elt_init = reshape_init_r (elt_type, d,
 				 /*first_initializer_p=*/NULL_TREE,
@@ -6670,6 +6672,7 @@ reshape_init_class (tree type, reshape_iter *d, bool first_initializer_p,
 	    }
 	  else if (TREE_CODE (d->cur->index) == IDENTIFIER_NODE)
 	    {
+	      CONSTRUCTOR_IS_DESIGNATED_INIT (new_init) = true;
 	      field = get_class_binding (type, d->cur->index);
 	      direct_desig = true;
 	    }
diff --git a/gcc/testsuite/g++.dg/cpp2a/desig26.C b/gcc/testsuite/g++.dg/cpp2a/desig26.C
new file mode 100644
index 000000000000..443fa3d089d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/desig26.C
@@ -0,0 +1,22 @@
+// PR c++/105925
+// { dg-do compile { target c++20 } }
+
+struct V
+{
+    int i;
+    double d;
+};
+
+struct X
+{
+    union
+    {
+        int x;
+        V y;
+    };
+};
+
+X foo()
+{
+    return {.y = {0, 0.0}};
+}