PR c++/81853 - using-directive and constexpr.

	* constexpr.c (cxx_eval_constant_expression): Handle USING_STMT.

From-SVN: r257762

gcc/cp/ChangeLog

 2018-02-16  Jason Merrill  <jason@redhat.com>
 
+	PR c++/81853 - using-directive and constexpr.
+	* constexpr.c (cxx_eval_constant_expression): Handle USING_STMT.
+
 	PR c++/84420 - ICE with structured binding in lambda.
 	* lambda.c (is_capture_proxy): Check DECL_DECOMPOSITION_P.
 

gcc/cp/constexpr.c

@@ -4726,6 +4726,10 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 					jump_target);
       break;
 
+    case USING_STMT:
+      r = void_node;
+      break;
+
     default:
       if (STATEMENT_CODE_P (TREE_CODE (t)))
 	{

gcc/testsuite/g++.dg/ext/stmtexpr22.C

+// PR c++/81853
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+namespace N {
+  enum { i };
+}
+
+int g ()
+{
+  constexpr int j = ({ using namespace N; i; });
+  return j;
+}