Pathes for Eclipse CDT are not well configured
Eclipse CDT is automatically configured by the tooling to find #include directives. For instance, in case of the simple message service, the map.hpp file can be included with
#include "simple_msgs/msg/map.hpp"
Since ROS 2 Humble, the installed files are nested in a relatively deep hierarchy; in the case of the example, map.hpp is in the folder install/simple_msgs/include/simple_msgs/simple_msgs/msg/ relative to your workspace. Before, there was a single simple_msgs folder within the include directory. In order to find the included header file, the search path install/simple_msgs/include/simple_msgs needs to be added.
The tooling currently does not configure the included paths correctly, resulting in header files that are not found and, in turn, undefined types from a CDT viewpoint. Even, if the code compiles, CDT marks errors and does not offer completion.