--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/desig8.C	Thu Oct 25 07:37:49 2018 +0900
@@ -0,0 +1,31 @@
+// PR c++/84874
+// { dg-do run { target c++17 } }
+// { dg-options "" }
+
+struct A { int a; struct { int b; }; };
+struct B { A d; };
+
+void
+foo (B *x)
+{
+  *x = { .d = { .b = 5 } };
+}
+
+void
+bar (A *x)
+{
+  *x = { .b = 6 };
+}
+
+int
+main ()
+{
+  B b = { { 2, 3 } };
+  foo (&b);
+  if (b.d.a != 0 || b.d.b != 5)
+    __builtin_abort ();
+  b.d.a = 8;
+  bar (&b.d);
+  if (b.d.a != 0 || b.d.b != 6)
+    __builtin_abort ();
+}