re PR c++/68449 (ICE in cxx_eval_constant_expression on atomic_load in C++)

	PR c++/68449
	* constexpr.c (cxx_eval_constant_expression): Handle NULL initializer.

	* g++.dg/pr68449.C: New.

From-SVN: r232176

gcc/cp/ChangeLog

+2016-01-08  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/68449
+	* constexpr.c (cxx_eval_constant_expression): Handle NULL initializer.
+
 2016-01-08  Jason Merrill  <jason@redhat.com>
 
 	* constexpr.c (cxx_eval_call_expression): Remove convert_to_void

gcc/cp/constexpr.c

@@ -3184,7 +3184,8 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	r = TARGET_EXPR_INITIAL (r);
       if (VAR_P (r))
 	if (tree *p = ctx->values->get (r))
-	  r = *p;
+	  if (*p != NULL_TREE)
+	    r = *p;
       if (DECL_P (r))
 	{
 	  if (!ctx->quiet)

gcc/testsuite/ChangeLog

+2016-01-08  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/68449
+	* g++.dg/pr68449.C: New.
+
 2016-01-08  Alan Lawrence  <alan.lawrence@arm.com>
 
 	PR tree-optimization/68707

gcc/testsuite/g++.dg/pr68449.C

+// PR c++/68449
+// { dg-do compile }
+// { dg-options "-Wsign-compare" }
+
+int
+foo (int a)
+{
+  return __extension__ ({ int b; b; }) < 0;
+}