std_msgs/sensors_msgs does not name a type

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Hi, I have a project with 2 files a main .cpp and a .hpp. The .cpp includes

#include "sensor_msgs/msg/nav_sat_fix.h"

and declares a variable

sensor_msgs::msg::NavSatFix n;

and this works just fine.

When I try to do the same thing by including sensor message in the .hpp file I get a colcon message stating that sensor_msgs does not name a type, even though I declared it identically to the .cpp reference.

My package.xml depends on sensor_msgs and my CMakeLists.txt finds sensor_msgs is listed as one of the ament_target_dependencies. The same thing happens if I include a std_msg in the .hpp section so I assume its some path missing somewhere, but I'm not sure why it works for the .cpp and not the .hpp.

UPDATE: Here is a simplified mini-project to show the issue I'm having

CMakeList.txt:

cmake_minimum_required(VERSION 3.5)
project(testing)

# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 14)
endif()

if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  add_compile_options(-Wall -Wextra -Wpedantic)
endif()

find_package(ament_cmake REQUIRED)
find_package(rclcpp REQUIRED)
find_package(std_msgs REQUIRED)
find_package(sensor_msgs REQUIRED)


include_directories("../dependencies/")

add_executable(${PROJECT_NAME} a.cpp)
ament_target_dependencies(${PROJECT_NAME} rclcpp std_msgs sensor_msgs obstacle_msgs)


install(TARGETS
  ${PROJECT_NAME}
  DESTINATION lib/${PROJECT_NAME}
)

ament_package()

package.xml:

<?xml version="1.0"?>
<package format="2">
  <name>testing</name>
  <version>0.0.0</version>
  <description>blah blah blah</description>

  <maintainer email="paul@somewhere.com">Paul</maintainer>
  <license>TODO</license>
  <!-- <author email="paul">Paul</author> -->

  <buildtool_depend>ament_cmake</buildtool_depend>
  <depend>rclcpp</depend>
  <depend>std_msgs</depend>
  <depend>sensor_msgs</depend>

  <!-- The export tag contains other, unspecified, tags -->
  <export>
    <!-- Other tools can request additional information be placed here -->
    <build_type>ament_cmake</build_type>
  </export>
</package>

a.cpp:

#include "b.hpp"

int main()
{
    B *b = new B();

    return 0;
}

b.hpp:

#ifndef SRC_TEST_B_HPP_
#define SRC_TEST_B_HPP_

#include "sensor_msgs/msg/nav_sat_fix.h"

class B {
public:
    sensor_msgs::msg::NavSatFix getPoly() { return m_polygon; }
private:
    sensor_msgs::msg::NavSatFix m_polygon;
};

#endif /* SRC_TEST_B_HPP_ */

Compiling output:

In file included from /home/paul/code/eclipse_ws/ROS2/src/testing/a.cpp:1:0:
/home/paul/code/eclipse_ws/ROS2/src/testing/b.hpp:8:2: error: ‘sensor_msgs’ does not name a type
  sensor_msgs::msg::NavSatFix getPoly() { return m_polygon; }
  ^~~~~~~~~~~
/home/paul/code/eclipse_ws/ROS2/src/testing/b.hpp:10:2: error: ‘sensor_msgs’ does not name a type
  sensor_msgs::msg::NavSatFix m_polygon;

What simple thing am I missing?

Sincerely, Paul.