c++: Add testcase for PR 90764.

     PR c++/90764
     * g++.dg/cpp1z/class-deduction69.C: New.

gcc/testsuite/g++.dg/cpp1z/class-deduction69.C

+// PR c++/90764
+// { dg-do compile { target c++17 } }
+
+namespace a {
+  struct b;
+  template <typename...> using c = b;
+}
+template <typename... d> struct e : a::c<d...> { // { dg-error "incomplete" }
+  using a::c<>::c;		// { dg-prune-output "not a base" }
+};
+template <template <typename> typename f> void g() { f(); }
+void h() { g<e>(); }