d: Fix assignment to anonymous union member corrupts sibling members in struct

gcc/d/ChangeLog:

	PR d/92309
	* types.cc (fixup_anonymous_offset): Don't set DECL_FIELD_OFFSET on
	anonymous fields.

gcc/testsuite/ChangeLog:

	PR d/92309
	* gdc.dg/pr92309.d: New test.

gcc/d/ChangeLog

 2020-03-16  Iain Buclaw  <ibuclaw@gdcproject.org>
 
+	PR d/92309
+	* types.cc (fixup_anonymous_offset): Don't set DECL_FIELD_OFFSET on
+	anonymous fields.
+
+2020-03-16  Iain Buclaw  <ibuclaw@gdcproject.org>
+
 	PR d/92216
 	* decl.cc (make_thunk): Don't set TREE_PUBLIC on thunks if the target
 	function is external to the current compilation.

gcc/d/types.cc

@@ -234,16 +234,20 @@ insert_aggregate_field (tree type, tree field, size_t offset)
 static void
 fixup_anonymous_offset (tree fields, tree offset)
 {
+  /* No adjustment in field offset required.  */
+  if (integer_zerop (offset))
+    return;
+
   while (fields != NULL_TREE)
     {
-      /* Traverse all nested anonymous aggregates to update their offset.
-	 Set the anonymous decl offset to its first member.  */
+      /* Traverse all nested anonymous aggregates to update the offset of their
+	 fields.  Note that the anonymous field itself is not adjusted, as it
+	 already has an offset relative to its outer aggregate.  */
       tree ftype = TREE_TYPE (fields);
       if (TYPE_NAME (ftype) && IDENTIFIER_ANON_P (TYPE_IDENTIFIER (ftype)))
 	{
 	  tree vfields = TYPE_FIELDS (ftype);
 	  fixup_anonymous_offset (vfields, offset);
-	  DECL_FIELD_OFFSET (fields) = DECL_FIELD_OFFSET (vfields);
 	}
       else
 	{

gcc/testsuite/ChangeLog

 2020-03-16  Iain Buclaw  <ibuclaw@gdcproject.org>
 
+	PR d/92309
+	* gdc.dg/pr92309.d: New test.
+
+2020-03-16  Iain Buclaw  <ibuclaw@gdcproject.org>
+
 	PR d/92216
 	* gdc.dg/imports/pr92216.d: New.
 	* gdc.dg/pr92216.d: New test.

gcc/testsuite/gdc.dg/pr92309.d

+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92309
+// { dg-do run { target hw } }
+// { dg-skip-if "needs gcc/config.d" { ! d_runtime } }
+
+union U
+{
+    struct
+    {
+        size_t a;
+        size_t b;
+        union
+        {
+            size_t c;
+            size_t d;
+        }
+    }
+}
+
+void main()
+{
+    U u;
+    assert(u.a == 0);
+    u.d = 1;
+    assert(u.a == 0);
+}