Don't neglect to "return *this".

llvm-svn: 165860
diff --git a/libcxx/include/future b/libcxx/include/future
index 758296a..39475bd 100644
--- a/libcxx/include/future
+++ b/libcxx/include/future
@@ -1872,6 +1872,7 @@
         __f_ = __f.__f_;
         __f.__f_ = nullptr;
     }
+    return *this;
 }
 
 template<class _Rp, class ..._ArgTypes>