Merge to upstream r226192.

Change-Id: Id459c8ce0d7dec371ff4d064ee2c71f6b7178e63
diff --git a/include/regex b/include/regex
index 7bc08a8..9a88983 100644
--- a/include/regex
+++ b/include/regex
@@ -2608,9 +2608,7 @@
         assign(_ForwardIterator __first, _ForwardIterator __last,
                             flag_type __f = regex_constants::ECMAScript)
         {
-            __member_init(__f);
-            __parse(__first, __last);
-            return *this;
+            return assign(basic_regex(__first, __last, __f));
         }
 
 #ifndef _LIBCPP_HAS_NO_GENERALIZED_INITIALIZERS