[PR c++/84702] ICE with default tmpl arg of overload set

https://gcc.gnu.org/ml/gcc-patches/2018-03/msg00227.html
	PR c++/84702
	* pt.c (process_template_arg): Mark lookup_keep on a default arg.

	PR c++/84702
	* g++.dg/lookup/pr84702.C: New.

From-SVN: r258258

gcc/cp/ChangeLog

+2018-03-05  Nathan Sidwell  <nathan@acm.org>
+
+	PR c++/84702
+	* pt.c (process_template_arg): Mark lookup_keep on a default arg.
+
 2018-03-05  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/84707

gcc/cp/pt.c

@@ -4425,6 +4425,9 @@ process_template_parm (tree list, location_t parm_loc, tree parm,
 
   pushdecl (decl);
 
+  if (defval && TREE_CODE (defval) == OVERLOAD)
+    lookup_keep (defval, true);
+  
   /* Build the parameter node linking the parameter declaration,
      its default argument (if any), and its constraints (if any). */
   parm = build_tree_list (defval, parm);

gcc/testsuite/ChangeLog

+2018-03-05  Nathan Sidwell  <nathan@acm.org>
+
+	PR c++/84702
+	* g++.dg/lookup/pr84702.C: New.
+
 2018-03-05  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/84564

gcc/testsuite/g++.dg/lookup/pr84702.C

+// PR c++/84702 failure to mark overload to keep
+// { dg-do compile { target c++11 } }
+
+void a ();
+
+namespace {
+  void a (int);
+}
+
+template<void (&b)() = a>
+void c () {
+  c ();
+}