add assert to notify incurred UB in case a node was linked during send_down

Author: menzels

lager/detail/nodes.hpp

@@ -218,16 +218,18 @@ class reader_node : public observable_reader_node<T>
             needs_send_down_ = false;
             needs_notify_    = true;
 
-            // We cannot use ranged-for here because children might
-            // change as a result of recompute(). This can invalidate
-            // the iterators on children, causing undefined behaviors.
-            // See also https://github.com/arximboldi/lager/pull/212
             const auto& children = this->children();
-            for (size_t i = 0, size = children.size(); i < size; ++i) {
-                if (auto child = children[i].lock()) {
+            const size_t n_children = children.size();
+            for (auto& wchild : children) {
+                if (auto child = wchild.lock()) {
                     child->send_down();
                 }
             }
+            assert(n_children == children.size() &&
+                   "Children must not change during send_down, \
+                    because iterators would be invalidated leading to UB. \
+                    Maybe you created a new cursor inside a cursor.map(...) \
+                    call?");
         }
     }