Node editor: when adding a new node, automatic links are created:

- from all selected nodes
- only connections between highest order socket types; so if there's RGBA
  and Value sockets, only RGBA sockets are connected. This because in
  these cases the Value sockets usually are for user input.
  Example: Mix node.

Thanks Trip for the hint!

source/blender/include/BSE_node.h

@@ -79,6 +79,8 @@ int node_has_hidden_sockets(struct bNode *node);
 struct bNode *node_add_node(struct SpaceNode *snode, int type, float locx, float locy);
 void node_adduplicate(struct SpaceNode *snode);
 
+void snode_autoconnect(struct SpaceNode *snode, struct bNode *node_to, int flag);
+
 /* ************* drawnode.c *************** */
 struct SpaceNode;
 struct bNodeLink;