Fix constexpr lvalue use of __real and __imag.

	* constexpr.c (potential_constant_expression_1): REALPART_EXPR and
	IMAGPART_EXPR can be lvalues.

From-SVN: r242349

gcc/cp/ChangeLog

 2016-11-12  Jason Merrill  <jason@redhat.com>
 
+	* constexpr.c (potential_constant_expression_1): REALPART_EXPR and
+	IMAGPART_EXPR can be lvalues.
+
 	DR 374
 	PR c++/56840
 	* pt.c (check_specialization_namespace): Allow any enclosing

gcc/cp/constexpr.c

@@ -5105,6 +5105,8 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict,
 #endif
       return RECUR (t, any);
 
+    case REALPART_EXPR:
+    case IMAGPART_EXPR:
     case COMPONENT_REF:
     case BIT_FIELD_REF:
     case ARROW_EXPR:
@@ -5276,8 +5278,6 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict,
 	return true;
       /* fall through.  */
 
-    case REALPART_EXPR:
-    case IMAGPART_EXPR:
     case CONJ_EXPR:
     case SAVE_EXPR:
     case FIX_TRUNC_EXPR:

gcc/testsuite/g++.dg/cpp0x/constexpr-complex2.C

+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+static _Complex int i;
+static_assert (&__imag i == &__imag i, "");