* pt.c (do_class_deduction): Handle 0 argument case.

From-SVN: r245665

gcc/cp/ChangeLog

+2017-02-22  Jason Merrill  <jason@redhat.com>
+
+	* pt.c (do_class_deduction): Handle 0 argument case.
+
 2017-02-22  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/79664

gcc/cp/pt.c

@@ -25126,6 +25126,14 @@ do_class_deduction (tree ptype, tree tmpl, tree init, int flags,
 
   if (cands == NULL_TREE)
     {
+      if (args->length() == 0)
+	{
+	  /* Try tmpl<>.  */
+	  tree t = lookup_template_class (tmpl, NULL_TREE, NULL_TREE,
+					  NULL_TREE, false, tf_none);
+	  if (t != error_mark_node)
+	    return t;
+	}
       error ("cannot deduce template arguments for %qT, as it has "
 	     "no deduction guides or user-declared constructors", type);
       return error_mark_node;

gcc/testsuite/g++.dg/cpp1z/class-deduction30.C

+// { dg-options -std=c++1z }
+
+template <class T = void> struct A { };
+
+A a{};
+