Implement insert_or_assign.

include/boost/unordered/detail/implementation.hpp

@@ -3704,6 +3704,27 @@ struct table_impl : boost::unordered::detail::table<Types>
         }
     }
 
+    template <typename Key, typename M>
+    emplace_return insert_or_assign_impl(
+        BOOST_FWD_REF(Key) k, BOOST_FWD_REF(M) obj)
+    {
+        std::size_t key_hash = this->hash(k);
+        node_pointer pos = this->find_node(key_hash, k);
+
+        if (pos) {
+            pos->value().second = boost::forward<M>(obj);
+            return emplace_return(iterator(pos), false);
+        } else {
+            return emplace_return(
+                iterator(this->resize_and_add_node(
+                    boost::unordered::detail::func::construct_node_pair(
+                        this->node_alloc(), boost::forward<Key>(k),
+                        boost::forward<M>(obj)),
+                    key_hash)),
+                true);
+        }
+    }
+
     ////////////////////////////////////////////////////////////////////////
     // Insert range methods
     //