c++: Add fixed test [PR96474]

Was happy to find out that my recent dguide fix (r11-7483) fixed
this test too.  In particular, the

+  /* Wait until the enclosing scope is non-dependent.  */
+  if (DECL_CLASS_SCOPE_P (tmpl)
+      && dependent_type_p (DECL_CONTEXT (tmpl)))
+    return ptype;

bit.

gcc/testsuite/ChangeLog:

	PR c++/96474
	* g++.dg/cpp1z/class-deduction83.C: New test.

gcc/testsuite/g++.dg/cpp1z/class-deduction83.C

@@ -0,0 +1,13 @@
+// PR c++/96474
+// { dg-do compile { target c++17 } }
+
+template <typename = void>
+struct A
+{
+    template <typename = void>
+    struct B
+    {
+    };
+};
+    
+A<>::B b;