PRIVATE option to ament_target_dependencies does not work

[felixf4xu]

#161 (comment)

PRIVATE and INTERFACE keywords aren't allowed.

I'd like to know why?

If only PUBLIC is allowed and it's used, I can only use public for all other dependencies, even for my PRIVATE library, which leads to a problem.

For example, visualization_msgs is my private library:

ament_target_dependencies(lanelet2_extension_lib PUBLIC
  visualization_msgs
)

but if I don't call

ament_export_dependencies(visualization_msgs) 

The user of my lib package will have this error:

The link interface of target "lanelet2_extension::lanelet2_extension_lib"
  contains:

    visualization_msgs::visualization_msgs__rosidl_generator_c

  but the target was not found.  Possible reasons include:

[clalancette]

I'd like to know why?

Nowadays, INTERFACE is also allowed, but PRIVATE still isn't.

Overall, we are (slowly) moving away from ament_target_dependencies, and instead just use target_link_libraries everywhere. This is one of the reasons; target_link_libraries can do everything that we need, and more. So my suggestion is that for these private dependencies, you switch to using target_link_libraries instead, and then you won't have to export them to downstream consumers.